Implement Standard Model

[cschwartz]

• Create app/models/standard.py with a Standard model using TimestampMixin + GoldSourceMixin + a composite index via GoldSourceMixin.gold_source_index("standard"). Fields: id, name, description, policy_id FK to Policy, metadata (JSON), timestamps.
• Configure cascade delete-orphan on the Policy → Standard relationship so deleting a Policy removes its Standards
• Define a SQLModel Relationship on Policy for navigating to its Standards
• Generate an Alembic migration for the Standard table
• Create app/routes/standards.py with endpoints: list standards (paginated, filterable by policy), get by ID, get by gold source
• Create a StandardFactory in tests/factories/standard.py
• Create tests: Standard is created with a Policy parent; creating a Standard without a valid Policy raises IntegrityError; deleting a Policy cascades to its Standards; gold source lookup works; route tests for listing, get by ID, get by gold source, and 404 handling

Done: Standard endpoints serve paginated lists, lookups by ID and gold source; Standard references its parent Policy via FK; creating a Standard without a valid Policy raises IntegrityError; deleting a Policy cascades to its Standards.