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/migrations/script.py.mako

@@ -0,0 +1,29 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+import sqlmodel  # noqa: F401 - autogenerate renders sqlmodel.* column types
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, Sequence[str], None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    ${downgrades if downgrades else "pass"}