feat: show shared DEMO-BUNSIN invite on signup for testers

Seed a reusable demo invite (ALLOW_DEMO_INVITE, default on) and surface it
on the Flutter signup screen with one-tap fill so others can try without
asking for a one-off admin mint.

Co-authored-by: okuma <o0kuma@users.noreply.github.com>

.env.example

@@ -5,6 +5,9 @@ GEMINI_API_KEY=
 # core-backend privileged endpoints (/invites, /admin/metrics, /admin/dashboard, /admin/push-test)
 ADMIN_API_TOKEN=
 
+# Shared demo invite DEMO-BUNSIN on signup UI (multiple testers). Set 0 in production.
+ALLOW_DEMO_INVITE=1
+
 # Optional: FCM legacy server key for real push delivery (escalation notify + /admin/push-test).
 # Without this, notifyUser soft-skips and records push_skipped metrics.
 FCM_SERVER_KEY=

core-backend/demo.go

@@ -0,0 +1,94 @@
+package main
+
+import (
+	"log"
+	"net/http"
+	"os"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+	"gorm.io/gorm"
+)
+
+// Shared closed-beta demo invite shown on the Flutter signup screen.
+// Multiple testers can use the same code when ALLOW_DEMO_INVITE is enabled.
+const demoInviteCode = "DEMO-BUNSIN"
+
+func demoInviteEnabled() bool {
+	v := strings.TrimSpace(os.Getenv("ALLOW_DEMO_INVITE"))
+	if v == "" {
+		// Default on for local / Phase-1 shared testing. Set ALLOW_DEMO_INVITE=0 in prod.
+		return true
+	}
+	switch strings.ToLower(v) {
+	case "0", "false", "no", "off":
+		return false
+	default:
+		return true
+	}
+}
+
+func isDemoInviteCode(code string) bool {
+	return strings.EqualFold(strings.TrimSpace(code), demoInviteCode)
+}
+
+func seedDemoInvite(db *gorm.DB) {
+	if !demoInviteEnabled() {
+		return
+	}
+	var existing InviteCode
+	err := db.Where("code = ?", demoInviteCode).First(&existing).Error
+	if err == nil {
+		return
+	}
+	if err != gorm.ErrRecordNotFound {
+		log.Printf("demo invite lookup: %v", err)
+		return
+	}
+	inv := InviteCode{
+		Code: demoInviteCode,
+		Note: "shared demo invite (ALLOW_DEMO_INVITE)",
+	}
+	if err := db.Create(&inv).Error; err != nil {
+		log.Printf("demo invite seed failed: %v", err)
+		return
+	}
+	log.Printf("seeded demo invite code %s", demoInviteCode)
+}
+
+func registerDemoRoutes(r *gin.Engine) {
+	r.GET("/demo", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{
+			"demo_invite_enabled": demoInviteEnabled(),
+			"demo_invite_code":    demoInviteCode,
+			"demo_display_name":   "테스터",
+			"hint":                "회원가입 화면에 표시된 테스트 코드를 그대로 쓰면 됩니다.",
+		})
+	})
+}
+
+// demoSignupInviteCode returns the InviteCode row for demo signups, creating it
+// if needed. Usability checks for "already used" are skipped by the caller.
+func demoSignupInviteCode(db *gorm.DB) (InviteCode, error) {
+	var invite InviteCode
+	err := db.Where("code = ?", demoInviteCode).First(&invite).Error
+	if err == nil {
+		return invite, nil
+	}
+	if err != gorm.ErrRecordNotFound {
+		return InviteCode{}, err
+	}
+	seedDemoInvite(db)
+	err = db.Where("code = ?", demoInviteCode).First(&invite).Error
+	return invite, err
+}
+
+// uniqueDemoUserInvite stores a per-user value on users.invite_code (unique)
+// while still accepting the shared DEMO-BUNSIN input.
+func uniqueDemoUserInvite() (string, error) {
+	suffix, err := generateInviteCode()
+	if err != nil {
+		return "", err
+	}
+	return demoInviteCode + "-" + suffix, nil
+}

core-backend/demo_test.go

@@ -0,0 +1,54 @@
+package main
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+	"testing"
+)
+
+func TestDemoInviteReusableForMultipleSignups(t *testing.T) {
+	t.Setenv("ALLOW_DEMO_INVITE", "1")
+	server, db := setupTestServer(t)
+	seedDemoInvite(db)
+
+	demoResp, err := http.Get(server.URL + "/demo")
+	if err != nil {
+		t.Fatalf("GET /demo: %v", err)
+	}
+	defer demoResp.Body.Close()
+	var demo map[string]any
+	if err := json.NewDecoder(demoResp.Body).Decode(&demo); err != nil {
+		t.Fatalf("decode demo: %v", err)
+	}
+	if demo["demo_invite_code"] != demoInviteCode {
+		t.Fatalf("demo code: %v", demo["demo_invite_code"])
+	}
+
+	a := postJSON(t, server.URL+"/auth/signup", signupRequest{
+		InviteCode:  demoInviteCode,
+		DisplayName: "테스터A",
+	})
+	defer a.Body.Close()
+	aBody, _ := io.ReadAll(a.Body)
+	if a.StatusCode != http.StatusOK {
+		t.Fatalf("first demo signup: %d %s", a.StatusCode, aBody)
+	}
+
+	b := postJSON(t, server.URL+"/auth/signup", signupRequest{
+		InviteCode:  demoInviteCode,
+		DisplayName: "테스터B",
+	})
+	defer b.Body.Close()
+	bBody, _ := io.ReadAll(b.Body)
+	if b.StatusCode != http.StatusOK {
+		t.Fatalf("second demo signup should reuse code: %d %s", b.StatusCode, bBody)
+	}
+
+	var outA, outB map[string]any
+	_ = json.Unmarshal(aBody, &outA)
+	_ = json.Unmarshal(bBody, &outB)
+	if outA["id"] == outB["id"] {
+		t.Fatalf("expected distinct users, got %#v %#v", outA, outB)
+	}
+}

core-backend/main.go

@@ -83,6 +83,7 @@ func setupRouter(db *gorm.DB, relay *ConnectionManager, ai *AIServiceClient) *gi
 	registerA1A2Routes(r, db)
 	registerBRoutes(r, db)
 	registerInviteOpsRoutes(r, db)
+	registerDemoRoutes(r)
 
 	r.GET("/admin/metrics", func(c *gin.Context) {
 		if !requireAdmin(c) {
@@ -162,7 +163,25 @@ func setupRouter(db *gorm.DB, relay *ConnectionManager, ai *AIServiceClient) *gi
 		// 초대 기반 베타(roadmap.md §2.6): 가입은 누군가 실제로 발급한 미사용
 		// 코드가 있어야만 된다 -- 아무 문자열이나 처음 쓰면 통과되던 이전
 		// 방식은 "초대 기반"이 아니었음.
+		//
+		// Shared demo: DEMO-BUNSIN (ALLOW_DEMO_INVITE, default on) can be reused
+		// by multiple testers; User.InviteCode still gets a unique stored value.
+		demo := demoInviteEnabled() && isDemoInviteCode(req.InviteCode)
 		var invite InviteCode
+		storedInvite := req.InviteCode
+		if demo {
+			var err error
+			invite, err = demoSignupInviteCode(db)
+			if err != nil {
+				c.JSON(http.StatusInternalServerError, gin.H{"detail": err.Error()})
+				return
+			}
+			storedInvite, err = uniqueDemoUserInvite()
+			if err != nil {
+				c.JSON(http.StatusInternalServerError, gin.H{"detail": err.Error()})
+				return
+			}
+		} else {
 			if err := db.Where("code = ?", req.InviteCode).First(&invite).Error; err != nil {
 				c.JSON(http.StatusBadRequest, gin.H{"detail": "invalid invite code"})
 				return
@@ -175,18 +194,21 @@ func setupRouter(db *gorm.DB, relay *ConnectionManager, ai *AIServiceClient) *gi
 				c.JSON(status, gin.H{"detail": detail})
 				return
 			}
+		}
 
-		user := User{InviteCode: req.InviteCode, DisplayName: req.DisplayName}
+		user := User{InviteCode: storedInvite, DisplayName: req.DisplayName}
 		if err := db.Create(&user).Error; err != nil {
 			c.JSON(http.StatusInternalServerError, gin.H{"detail": err.Error()})
 			return
 		}
 		db.Create(&TwinSettings{UserID: user.ID, AutonomyLevel: AutonomyL0})
 
+		if !demo {
 			now := time.Now()
 			invite.UsedAt = &now
 			invite.UsedByUserID = &user.ID
 			db.Save(&invite)
+		}
 
 		session, err := createSession(db, user.ID)
 		if err != nil {
@@ -674,6 +696,7 @@ func main() {
 		return
 	}
 	db := openDB()
+	seedDemoInvite(db)
 	relay := newConnectionManager()
 	ai := newAIServiceClient()
 	r := setupRouter(db, relay, ai)

mobile/lib/config.dart

@@ -8,6 +8,12 @@ class AppConfig {
     defaultValue: 'http://10.0.2.2:8080', // Android emulator → host localhost
   );
 
+  /// Shared demo invite (must match core-backend `demoInviteCode`).
+  /// Shown on the signup screen so other testers can join without admin mint.
+  static const demoInviteCode = 'DEMO-BUNSIN';
+  static const demoDisplayName = '테스터';
+
+
   static String wsBase() {
     final uri = Uri.parse(coreApiBase);
     final scheme = uri.scheme == 'https' ? 'wss' : 'ws';

mobile/lib/screens/signup_screen.dart

@@ -1,6 +1,8 @@
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 import 'package:provider/provider.dart';
 
+import '../config.dart';
 import '../state/session_state.dart';
 import '../theme/app_theme.dart';
 import '../widgets/twin_hero_backdrop.dart';
@@ -23,6 +25,12 @@ class _SignupScreenState extends State<SignupScreen> {
     super.dispose();
   }
 
+  void _fillDemoCredentials() {
+    _invite.text = AppConfig.demoInviteCode;
+    _name.text = AppConfig.demoDisplayName;
+    setState(() {});
+  }
+
   @override
   Widget build(BuildContext context) {
     final session = context.watch<SessionState>();
@@ -72,9 +80,23 @@ class _SignupScreenState extends State<SignupScreen> {
                     style: theme.textTheme.bodyMedium?.copyWith(color: scheme.onSurfaceVariant),
                   ),
                 ),
-                const Spacer(flex: 3),
+                const Spacer(flex: 2),
                 TwinFadeUp(
-                  delay: const Duration(milliseconds: 220),
+                  delay: const Duration(milliseconds: 200),
+                  child: _DemoTestPanel(
+                    onFill: _fillDemoCredentials,
+                    onCopy: () async {
+                      await Clipboard.setData(const ClipboardData(text: AppConfig.demoInviteCode));
+                      if (!context.mounted) return;
+                      ScaffoldMessenger.of(context).showSnackBar(
+                        const SnackBar(content: Text('테스트 초대 코드를 복사했습니다')),
+                      );
+                    },
+                  ),
+                ),
+                const SizedBox(height: 16),
+                TwinFadeUp(
+                  delay: const Duration(milliseconds: 240),
                   child: Column(
                     crossAxisAlignment: CrossAxisAlignment.stretch,
                     children: [
@@ -133,6 +155,68 @@ class _SignupScreenState extends State<SignupScreen> {
   }
 }
 
+/// Visible test credentials so other people can try the closed beta without
+/// asking Master for a one-off invite mint.
+class _DemoTestPanel extends StatelessWidget {
+  const _DemoTestPanel({required this.onFill, required this.onCopy});
+
+  final VoidCallback onFill;
+  final VoidCallback onCopy;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    return Material(
+      color: TwinTokens.mist.withValues(alpha: 0.85),
+      borderRadius: BorderRadius.circular(16),
+      child: InkWell(
+        onTap: onFill,
+        borderRadius: BorderRadius.circular(16),
+        child: Padding(
+          padding: const EdgeInsets.fromLTRB(16, 14, 12, 14),
+          child: Column(
+            crossAxisAlignment: CrossAxisAlignment.start,
+            children: [
+              Text(
+                '테스트용 (누구나)',
+                style: theme.textTheme.labelLarge?.copyWith(
+                  color: TwinTokens.forest,
+                  letterSpacing: 0.4,
+                ),
+              ),
+              const SizedBox(height: 6),
+              Row(
+                children: [
+                  Expanded(
+                    child: Text(
+                      '초대 코드  ${AppConfig.demoInviteCode}\n표시 이름  ${AppConfig.demoDisplayName}',
+                      style: theme.textTheme.titleSmall?.copyWith(
+                        color: TwinTokens.ink,
+                        height: 1.45,
+                        fontWeight: FontWeight.w700,
+                      ),
+                    ),
+                  ),
+                  IconButton(
+                    tooltip: '코드 복사',
+                    onPressed: onCopy,
+                    icon: const Icon(Icons.copy_rounded, size: 20),
+                  ),
+                ],
+              ),
+              const SizedBox(height: 4),
+              Text(
+                '탭하면 입력란에 채워집니다 · 여러 명이 같은 코드로 가입 가능',
+                style: theme.textTheme.bodySmall?.copyWith(color: TwinTokens.ink.withValues(alpha: 0.55)),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}
+
 class _PressScale extends StatefulWidget {
   const _PressScale({required this.child});