feat(backend): add FastAPI backend — procurement, inventory domain, Casdoor auth, PM arrival endpoints

backend/app/db/engine.py

@@ -0,0 +1,29 @@
+from collections.abc import AsyncGenerator
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+from app.config import settings
+
+engine = create_async_engine(
+    settings.DATABASE_URL,
+    echo=settings.DEBUG_SQL,
+    pool_pre_ping=True,
+)
+
+AsyncSessionLocal = async_sessionmaker(engine, expire_on_commit=False)
+
+
+async def get_db() -> AsyncGenerator[AsyncSession, None]:
+    """FastAPI dependency: yields a session, committing on success.
+
+    Services that need multi-statement atomicity use ``async with db.begin()``
+    blocks (those commit/rollback explicitly); a plain commit here persists
+    the single-statement writes of the CRUD routers.
+    """
+    async with AsyncSessionLocal() as session:
+        try:
+            yield session
+            await session.commit()
+        except Exception:
+            await session.rollback()
+            raise