feat(backend): db session config

backend/src/db/database.py

@@ -0,0 +1,40 @@
+from collections.abc import AsyncGenerator
+from typing import Any
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+from src.config import get_settings
+
+settings = get_settings()
+
+# Build engine kwargs conditionally (SQLite doesn't support pool settings)
+engine_kwargs: dict[str, Any] = {
+    "echo": settings.debug,
+}
+
+if settings.database_url.startswith("postgresql"):
+    engine_kwargs.update(
+        pool_pre_ping=True,
+        pool_size=5,
+        max_overflow=10,
+        pool_recycle=3600,
+    )
+
+engine = create_async_engine(settings.database_url, **engine_kwargs)
+
+async_session_factory = async_sessionmaker(
+    engine,
+    class_=AsyncSession,
+    expire_on_commit=False,
+)
+
+
+async def get_db() -> AsyncGenerator[AsyncSession, None]:
+    """Dependency that provides a database session."""
+    async with async_session_factory() as session:
+        try:
+            yield session
+            await session.commit()
+        except Exception:
+            await session.rollback()
+            raise