Initial commit: clean project structure

- Backend: FastAPI + SQLAlchemy + Celery (Python 3.11+)
- Frontend: Vue 3 + TypeScript + Pinia + Tailwind
- Admin Frontend: separate Vue 3 app for management
- Docker Compose: 9 services orchestration
- Specs: design prototypes, memory system PRD, product roadmap

Cleanup performed:
- Removed temporary debug scripts from backend root
- Removed deprecated admin_app.py (embedded UI)
- Removed duplicate docs from admin-frontend
- Updated .gitignore for Vite cache and egg-info

backend/app/db/database.py

@@ -0,0 +1,50 @@
+import threading
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+from app.core.config import settings
+
+_engine = None
+_session_factory: async_sessionmaker[AsyncSession] | None = None
+_lock = threading.Lock()
+
+
+def _get_engine():
+    global _engine
+    if _engine is None:
+        with _lock:
+            if _engine is None:
+                _engine = create_async_engine(
+                    settings.database_url,
+                    echo=settings.debug,
+                    pool_pre_ping=True,
+                    pool_recycle=300,
+                )
+    return _engine
+
+
+def _get_session_factory():
+    global _session_factory
+    if _session_factory is None:
+        with _lock:
+            if _session_factory is None:
+                _session_factory = async_sessionmaker(
+                    _get_engine(), class_=AsyncSession, expire_on_commit=False
+                )
+    return _session_factory
+
+
+async def init_db():
+    """Create tables if they do not exist."""
+    from app.db.models import Base  # main models
+
+    engine = _get_engine()
+    async with engine.begin() as conn:
+        await conn.run_sync(Base.metadata.create_all)
+
+
+async def get_db():
+    """Yield a DB session with proper cleanup."""
+    session_factory = _get_session_factory()
+    async with session_factory() as session:
+        yield session