Ports the Python prototype (backend/) to the actual chosen stack --
Gin + gorilla/websocket + GORM, same DB schema (models.go mirrors
backend/app/models.py), same endpoints (signup, message send,
WebSocket relay). backend/ stays as a reference prototype, not
removed.
Verified with go test: signup, duplicate-invite-code rejection (409),
404 on an unknown conversation, and WebSocket broadcast delivery all
pass -- the same cases the Python version was checked against.
Push notifications, AI service integration, and multi-device sync
are not in this commit -- see core-backend/README.md.
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