Phase 0 milestone 0.5: Registrant vertical slice

Registrant model + schemas + router + migration 0003, exercising the
vendored framework end-to-end (CRUDService, RBAC, audit, version history).

- SSN app-layer encrypted (AES-256-GCM, HKDF key, column-bound AAD,
  versioned format) in security/column_crypto.py; write-only on the model;
  serializer exposes only ssn_masked (XXX-XX-1234). C.5.7.50 / SC-28.
- pii_read audit on GET, list (one row per page with ids), and history.
  C.5.6.9 #5 / AU-14.
- Portal-only callers (YOUTH/PARENT/HOST) limited to own row via
  Registrant.user_id; out-of-scope is 404. PARENT_USER sees nothing until
  Guardian lands. History gated on VersionHistory, not View.
- minor_flag derived from dob at read time.

Framework changes (flag to OWC on re-vendor):
- crud._to_dict honors __audit_redact__, keeping ciphertext out of
  audit_log and version_history.
- crud._to_dict serializes date (not only datetime) for JSON columns.
- CRUDService.history annotation uses builtins.list (method shadowed it).

98 tests passing, 94.83% coverage; ruff + mypy clean.
Coverage trace at docs/coverage/registrant.md.

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

tests/conftest.py

@@ -39,6 +39,7 @@ def db() -> Iterator[Session]:
     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
     from ypms.db import Base
 
     # StaticPool + check_same_thread=False share the same in-memory DB across