diff --git a/.gitignore b/.gitignore
index 37de000..7152287 100644
--- a/.gitignore
+++ b/.gitignore
@@ -11,3 +11,6 @@ poc/tone-corpus/data/
 
 __pycache__/
 *.pyc
+
+# Local dev DB (backend/, SQLite fallback for DATABASE_URL)
+*.db
diff --git a/backend/README.md b/backend/README.md
new file mode 100644
index 0000000..00e89f2
--- /dev/null
+++ b/backend/README.md
@@ -0,0 +1,39 @@
+# 분신 backend
+
+Phase 1 백엔드 인프라 뼈대 (`docs/roadmap.md` Phase 1 §2.1). 스택 결정은
+`docs/tech-design.md` §8 참고 — Python/FastAPI, PostgreSQL(프로덕션)/SQLite(로컬 개발),
+WebSocket 릴레이.
+
+## 실행
+
+```bash
+pip install -r requirements.txt
+uvicorn app.main:app --reload
+```
+
+기본은 `sqlite:///./dev.db`로 뜬다. 프로덕션 DB를 쓰려면:
+
+```bash
+export DATABASE_URL=postgresql+psycopg2://user:pass@host/dbname
+```
+
+## 지금 있는 것 (2.1 백엔드 인프라 뼈대)
+
+- `GET /health` — 헬스체크
+- `POST /auth/signup` — 초대 코드 기반 가입 (중복 코드는 409)
+- `POST /conversations/{id}/messages` — 메시지 저장 + 같은 대화방 WebSocket 커넥션에 브로드캐스트
+- `WS /ws/conversations/{id}` — 대화방별 실시간 릴레이 (인메모리 커넥션 매니저)
+- DB 모델 (`app/models.py`): `users`, `contacts`, `conversations`,
+  `conversation_participants`, `messages`, `twin_settings`, `whitelist_rules`,
+  `escalation_logs` — `roadmap.md` Phase 1 §2.1 스키마 그대로
+
+signup/message-send/404/WebSocket 브로드캐스트까지 `TestClient`로 실제 실행해서 확인함
+(테스트 스크립트는 커밋 안 함 — 필요하면 정식 `tests/`로 다시 만들 것).
+
+## 아직 없는 것 (다음 워크스트림)
+
+- 2.2 AI 파이프라인 연동 — `poc/tone-corpus/generate_draft.py` 등을 여기 API로 이식
+- 푸시 알림 연동
+- 인증 토큰/세션 (지금은 invite_code로 가입만 되고 로그인 세션 개념이 없음)
+- 프로덕션 마이그레이션 도구 (지금은 `Base.metadata.create_all`로 스타트업 시 테이블 생성 —
+  Alembic 같은 마이그레이션은 스키마가 안정되면 도입)
diff --git a/backend/app/__init__.py b/backend/app/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/backend/app/config.py b/backend/app/config.py
new file mode 100644
index 0000000..25296fb
--- /dev/null
+++ b/backend/app/config.py
@@ -0,0 +1,8 @@
+import os
+
+# Production: postgresql+psycopg2://user:pass@host/dbname (tech-design.md §8).
+# Defaults to a local SQLite file so the app runs without a Postgres instance
+# during development/testing.
+DATABASE_URL = os.environ.get("DATABASE_URL", "sqlite:///./dev.db")
+
+GEMINI_API_KEY = os.environ.get("GEMINI_API_KEY")
diff --git a/backend/app/db.py b/backend/app/db.py
new file mode 100644
index 0000000..b4cc0e2
--- /dev/null
+++ b/backend/app/db.py
@@ -0,0 +1,17 @@
+from sqlalchemy import create_engine
+from sqlalchemy.orm import declarative_base, sessionmaker
+
+from .config import DATABASE_URL
+
+connect_args = {"check_same_thread": False} if DATABASE_URL.startswith("sqlite") else {}
+engine = create_engine(DATABASE_URL, connect_args=connect_args)
+SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+Base = declarative_base()
+
+
+def get_db():
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()
diff --git a/backend/app/main.py b/backend/app/main.py
new file mode 100644
index 0000000..010b31a
--- /dev/null
+++ b/backend/app/main.py
@@ -0,0 +1,103 @@
+from typing import Dict, List
+
+from fastapi import Depends, FastAPI, HTTPException, WebSocket, WebSocketDisconnect
+from pydantic import BaseModel
+from sqlalchemy.orm import Session
+
+from . import models
+from .db import Base, engine, get_db
+
+app = FastAPI(title="분신 backend")
+
+
+@app.on_event("startup")
+def create_tables():
+    Base.metadata.create_all(bind=engine)
+
+
+@app.get("/health")
+def health():
+    return {"status": "ok"}
+
+
+class SignupRequest(BaseModel):
+    invite_code: str
+    display_name: str
+
+
+@app.post("/auth/signup")
+def signup(req: SignupRequest, db: Session = Depends(get_db)):
+    existing = db.query(models.User).filter_by(invite_code=req.invite_code).first()
+    if existing:
+        raise HTTPException(status_code=409, detail="invite_code already used")
+    user = models.User(invite_code=req.invite_code, display_name=req.display_name)
+    db.add(user)
+    db.commit()
+    db.refresh(user)
+    db.add(models.TwinSettings(user_id=user.id))
+    db.commit()
+    return {"id": user.id, "display_name": user.display_name}
+
+
+class SendMessageRequest(BaseModel):
+    sender_id: int
+    text: str
+    sender_mode: models.SenderMode = models.SenderMode.HUMAN
+
+
+@app.post("/conversations/{conversation_id}/messages")
+async def send_message(conversation_id: int, req: SendMessageRequest, db: Session = Depends(get_db)):
+    conversation = db.query(models.Conversation).get(conversation_id)
+    if not conversation:
+        raise HTTPException(status_code=404, detail="conversation not found")
+    message = models.Message(
+        conversation_id=conversation_id,
+        sender_id=req.sender_id,
+        sender_mode=req.sender_mode,
+        text=req.text,
+    )
+    db.add(message)
+    db.commit()
+    db.refresh(message)
+    await relay.broadcast(conversation_id, {
+        "id": message.id,
+        "sender_id": message.sender_id,
+        "sender_mode": message.sender_mode.value,
+        "text": message.text,
+    })
+    return {"id": message.id}
+
+
+class ConnectionManager:
+    """In-memory WebSocket fan-out per conversation. Fine for a small closed
+    beta (roadmap.md Phase 1); revisit if the relay needs to scale past one
+    process."""
+
+    def __init__(self):
+        self.connections: Dict[int, List[WebSocket]] = {}
+
+    async def connect(self, conversation_id: int, websocket: WebSocket):
+        await websocket.accept()
+        self.connections.setdefault(conversation_id, []).append(websocket)
+
+    def disconnect(self, conversation_id: int, websocket: WebSocket):
+        conns = self.connections.get(conversation_id, [])
+        if websocket in conns:
+            conns.remove(websocket)
+
+    async def broadcast(self, conversation_id: int, payload: dict):
+        for ws in self.connections.get(conversation_id, []):
+            await ws.send_json(payload)
+
+
+relay = ConnectionManager()
+
+
+@app.websocket("/ws/conversations/{conversation_id}")
+async def conversation_socket(websocket: WebSocket, conversation_id: int):
+    await relay.connect(conversation_id, websocket)
+    try:
+        while True:
+            await websocket.receive_text()
+    except WebSocketDisconnect:
+        relay.disconnect(conversation_id, websocket)
diff --git a/backend/app/models.py b/backend/app/models.py
new file mode 100644
index 0000000..361d7c4
--- /dev/null
+++ b/backend/app/models.py
@@ -0,0 +1,128 @@
+import enum
+
+from sqlalchemy import (
+    Boolean,
+    Column,
+    DateTime,
+    Enum,
+    ForeignKey,
+    Integer,
+    String,
+    func,
+)
+from sqlalchemy.orm import relationship
+
+from .db import Base
+
+
+class AutonomyLevel(str, enum.Enum):
+    L0 = "L0"
+    L1 = "L1"
+    L2 = "L2"
+
+
+class SenderMode(str, enum.Enum):
+    HUMAN = "human"
+    TWIN = "twin"
+
+
+class User(Base):
+    __tablename__ = "users"
+
+    id = Column(Integer, primary_key=True)
+    invite_code = Column(String, unique=True, nullable=False, index=True)
+    display_name = Column(String, nullable=False)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+
+    twin_settings = relationship("TwinSettings", back_populates="user", uselist=False)
+
+
+class Contact(Base):
+    """A relationship between the owner and a counterpart -- carries
+    per-peer state like the veto flag (tech-design.md §4:
+    twin_disabled_by_peer), independent of any one conversation."""
+
+    __tablename__ = "contacts"
+
+    id = Column(Integer, primary_key=True)
+    owner_user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+    contact_user_id = Column(Integer, ForeignKey("users.id"), nullable=True)
+    display_name = Column(String, nullable=False)
+    relationship_note = Column(String, nullable=True)
+    twin_disabled_by_peer = Column(Boolean, nullable=False, default=False)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+
+
+class Conversation(Base):
+    __tablename__ = "conversations"
+
+    id = Column(Integer, primary_key=True)
+    is_group = Column(Boolean, nullable=False, default=False)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+
+    participants = relationship("ConversationParticipant", back_populates="conversation")
+    messages = relationship("Message", back_populates="conversation")
+
+
+class ConversationParticipant(Base):
+    __tablename__ = "conversation_participants"
+
+    id = Column(Integer, primary_key=True)
+    conversation_id = Column(Integer, ForeignKey("conversations.id"), nullable=False)
+    user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+
+    conversation = relationship("Conversation", back_populates="participants")
+
+
+class Message(Base):
+    __tablename__ = "messages"
+
+    id = Column(Integer, primary_key=True)
+    conversation_id = Column(Integer, ForeignKey("conversations.id"), nullable=False)
+    sender_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+    sender_mode = Column(Enum(SenderMode), nullable=False, default=SenderMode.HUMAN)
+    text = Column(String, nullable=False)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+
+    conversation = relationship("Conversation", back_populates="messages")
+
+
+class TwinSettings(Base):
+    __tablename__ = "twin_settings"
+
+    id = Column(Integer, primary_key=True)
+    user_id = Column(Integer, ForeignKey("users.id"), unique=True, nullable=False)
+    autonomy_level = Column(Enum(AutonomyLevel), nullable=False, default=AutonomyLevel.L0)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+
+    user = relationship("User", back_populates="twin_settings")
+
+
+class WhitelistRule(Base):
+    """L2 auto-send whitelist -- a (contact, topic) pair the owner has
+    approved for unattended replies. contact_id null = applies to any
+    counterpart (PRD.md §3.1)."""
+
+    __tablename__ = "whitelist_rules"
+
+    id = Column(Integer, primary_key=True)
+    user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+    contact_id = Column(Integer, ForeignKey("contacts.id"), nullable=True)
+    topic_keyword = Column(String, nullable=False)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+
+
+class EscalationLog(Base):
+    """One row per escalation_filter.py trigger -- the post-hoc notification
+    + undo trail required by AGENTS.md's absolute safety invariants."""
+
+    __tablename__ = "escalation_logs"
+
+    id = Column(Integer, primary_key=True)
+    user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+    conversation_id = Column(Integer, ForeignKey("conversations.id"), nullable=False)
+    reason = Column(String, nullable=False)
+    message_snippet = Column(String, nullable=False)
+    resolved = Column(Boolean, nullable=False, default=False)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
diff --git a/backend/requirements.txt b/backend/requirements.txt
new file mode 100644
index 0000000..be191dd
--- /dev/null
+++ b/backend/requirements.txt
@@ -0,0 +1,6 @@
+fastapi>=0.115.0
+uvicorn[standard]>=0.30.0
+sqlalchemy>=2.0.0
+psycopg2-binary>=2.9.0
+websockets>=13.0
+pydantic>=2.0.0
diff --git a/docs/roadmap.md b/docs/roadmap.md
index af54a6c..e4654d6 100644
--- a/docs/roadmap.md
+++ b/docs/roadmap.md
@@ -33,9 +33,11 @@
 #### 2. 워크스트림별 작업
 
 **2.1 백엔드 인프라** (PoC 결과 무관 — 지금 착수 가능)
-- [ ] 계정/인증 (초대 코드 기반 가입)
-- [ ] 메시지 릴레이 서버 (송수신, 멀티 디바이스 동기화)
-- [ ] DB 스키마: users, contacts, conversations, messages, twin_settings, escalation_logs, whitelist_rules
+- [x] 계정/인증 (초대 코드 기반 가입) — `backend/app/main.py` `POST /auth/signup`, 중복 코드 409 확인함
+- [x] 메시지 릴레이 서버 (송수신) — `backend/app/main.py` WebSocket + REST, 실제 TestClient로 브로드캐스트 확인함.
+  멀티 디바이스 동기화(같은 유저 여러 기기)는 아직 — 지금은 대화방 단위 인메모리 커넥션 매니저뿐
+- [x] DB 스키마: users, contacts, conversations, messages, twin_settings, escalation_logs, whitelist_rules
+  — `backend/app/models.py`
 - [ ] 푸시 알림 서비스 연동
 
 **2.2 AI 파이프라인 프로덕션화** (PoC 스크립트 → 서비스로 승격)
@@ -76,7 +78,8 @@
 #### 4. 권장 착수 순서 (진행 상황)
 
 1. [x] §1 기술 스택 결정
-2. [ ] 2.1 백엔드 기본 인프라 + 2.3 채팅 UI 뼈대 (병행) — **다음 작업**
+2. [~] 2.1 백엔드 기본 인프라 (계정/인증, DB 스키마, 메시지 릴레이 — `backend/` 완료, 푸시 알림만 남음)
+   + 2.3 채팅 UI 뼈대 (병행) — **안드로이드 클라이언트 쪽이 다음 작업**
 3. [ ] 2.2 AI 파이프라인 프로덕션화 (PoC 스크립트 재사용)
 4. [ ] 2.3 나머지 UX(온보딩·설정·뱃지)
 5. [ ] 2.4/2.5 안전장치·QA