Implement Statement Model

[cschwartz]

• Create app/models/statement.py with a Statement model using TimestampMixin + GoldSourceMixin + a composite index via GoldSourceMixin.gold_source_index("statement"). Fields: id, name, description, standard_id FK to Standard, metadata (JSON), timestamps.
• Configure cascade delete-orphan on the Standard → Statement relationship so deleting a Standard removes its Statements
• Define a SQLModel Relationship on Standard for navigating to its Statements
• Generate an Alembic migration for the Statement table
• Create app/routes/statements.py with endpoints: list statements (paginated, filterable by standard), get by ID, get by gold source
• Create a StatementFactory in tests/factories/statement.py
• Create tests: full Policy → Standard → Statement hierarchy can be created and navigated; cascade delete from Policy removes Standards and Statements; get_by_gold_source() works at each level of the hierarchy; route tests for listing, get by ID, get by gold source, and 404 handling

Done: Statement endpoints serve paginated lists, lookups by ID and gold source; full Policy → Standard → Statement hierarchy can be created and navigated; cascade delete from Policy removes Standards and Statements; gold source lookup works at each level.