feat: db base + config tests

Co-Authored-By: Claude <noreply@anthropic.com>

yunke-server/app/db/base.py

@@ -0,0 +1,48 @@
+"""Database core: declarative base, async engine, session factory.
+
+Production targets MySQL via `asyncmy` (configured by `Settings.db_url`).
+Unit tests build their own in-memory SQLite engines (see `tests/test_db_base.py`)
+and only import `Base` / `async_session_maker` from here.
+"""
+
+from collections.abc import AsyncIterator
+
+from sqlalchemy.ext.asyncio import (
+    AsyncSession,
+    async_sessionmaker,
+    create_async_engine,
+)
+from sqlalchemy.orm import DeclarativeBase
+
+from app.config import get_settings
+
+
+class Base(DeclarativeBase):
+    """Common declarative base for all ORM models in the project."""
+
+
+settings = get_settings()
+
+async_engine = create_async_engine(
+    settings.db_url,
+    pool_pre_ping=True,
+    future=True,
+)
+
+async_session_maker: async_sessionmaker[AsyncSession] = async_sessionmaker(
+    async_engine,
+    expire_on_commit=False,
+)
+
+
+async def get_session() -> AsyncIterator[AsyncSession]:
+    """FastAPI dependency that yields an `AsyncSession`.
+
+    Usage::
+
+        @app.get("/")
+        async def index(session: AsyncSession = Depends(get_session)):
+            ...
+    """
+    async with async_session_maker() as session:
+        yield session