Phase 0 milestone 0.7: retention scaffold

Every mapped table resolves a retention policy; a report-only job runs
over all of them and logs "N records to purge". ARCH §6.4; NIST AU-11,
SI-12; source RFP C.5.7.42.

- framework/retention.py: RetentionPolicy (data class, disposal, period,
  anchor column, cadence, approved, authority), run_retention(), and a
  `python -m ypms.framework.retention` entrypoint. Phase 0 never mutates
  data: interim-period rows are counted (interim_eligible), approved-period
  rows are counted (to_purge), nothing is purged. One retention_run audit
  row per run.
- retention_classes.py: PIA §9 data classes at their interim periods, all
  approved=False pending YPMS-POAM-054. Classes without a PIA basis
  (hosts, operational logs, accounts, consent-derived) are unset and logged
  by name with their governing POA&M item. Vendored framework tables are
  mapped by table name so vendored files stay unchanged.
- stub_models.py + migration 0004: id + mixin models for the 26 stub
  entities so each can carry a policy. Migration uses a literal table list.
- ypms/models.py: single import-all module used by Alembic env, the job,
  and the test fixtures.
- Drift guard: test upgrades a scratch SQLite DB to head and compares to
  Base.metadata. Found pre-existing drift in 0002 (named unique
  constraints duplicating the unique indexes on roles.code, users.username,
  users.email); allow-listed, follow-up noted in PHASE0_STATUS.

156 tests passing; ruff + mypy clean.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/conftest.py

@@ -36,11 +36,7 @@ def db() -> Iterator[Session]:
     Import happens inside the fixture so the model modules register themselves
     on Base.metadata before create_all runs.
     """
-    import ypms.audit.models  # noqa: F401
-    import ypms.framework.version_history  # noqa: F401
-    import ypms.security.models  # noqa: F401
-    import ypms.users.models  # noqa: F401
-    import ypms.youth.models  # noqa: F401
+    import ypms.models  # noqa: F401
     from ypms.db import Base
 
     # StaticPool + check_same_thread=False share the same in-memory DB across