feat: merge cartsnitch/api into api/ subdirectory

Consolidate API gateway service into monorepo.
Squashed from https://github.com/cartsnitch/api main (89bacb1).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/cartsnitch_api/database.py

@@ -0,0 +1,16 @@
+"""Database session management for the API gateway."""
+
+from collections.abc import AsyncGenerator
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+from cartsnitch_api.config import settings
+
+engine = create_async_engine(settings.database_url, echo=False)
+async_session_factory = async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)
+
+
+async def get_db() -> AsyncGenerator[AsyncSession, None]:
+    """FastAPI dependency that yields an async DB session."""
+    async with async_session_factory() as session:
+        yield session