feat: flow engine data layer with tests

- SQLModel tables: flow_definitions/versions/runs, approval_tasks
- Alembic setup with autogenerate (sqlmodel types fixed via script.py.mako)
- Settings extended (shared root .env), db engine/session helpers
- pytest suite: conftest with isolated test DB, 5 model tests green

backend/app/db.py

@@ -0,0 +1,36 @@
+"""Database engine, sessions, and FastAPI dependency."""
+
+from collections.abc import Iterator
+
+from sqlalchemy import Engine
+from sqlmodel import Session, SQLModel, create_engine
+
+from .settings import settings
+
+_engine: Engine | None = None
+
+
+def get_engine(url: str | None = None) -> Engine:
+    """Process-wide engine singleton (override URL in tests via settings)."""
+    global _engine
+    if _engine is None:
+        _engine = create_engine(url or settings.database_url)
+    return _engine
+
+
+def reset_engine() -> None:
+    """Drop the cached engine (used by the test suite between DB targets)."""
+    global _engine
+    if _engine is not None:
+        _engine.dispose()
+        _engine = None
+
+
+def get_session() -> Iterator[Session]:
+    with Session(get_engine()) as session:
+        yield session
+
+
+def create_all(url: str | None = None) -> None:
+    """Create tables directly (dev convenience; migrations own prod schema)."""
+    SQLModel.metadata.create_all(get_engine(url))