feat: add SQLAlchemy database layer with get_db dependency

tests/test_database.py

@@ -0,0 +1,16 @@
+from sqlalchemy.orm import Session
+from app.core.database import Base, get_db
+
+
+def test_get_db_yields_session():
+    gen = get_db()
+    db = next(gen)
+    assert isinstance(db, Session)
+    try:
+        next(gen)
+    except StopIteration:
+        pass
+
+
+def test_base_has_metadata():
+    assert Base.metadata is not None