feat(ttp): E.1.9 API contract — seven router endpoints, admin-gated state mutations, response models

Mounts /api/v1/ttp/* with empty-list / empty-Navigator responses.
GET endpoints viewer-gated; POST/DELETE /rules/{rule_id}/state
admin-gated server-side. POST parses JSON manually so a malformed
body returns the documented 400 (per feedback_schemathesis_400).

Drops xfail-strict markers from E.2.8 tests now that the router is
mounted; 26 tests pass against the contract handlers.

tests/api/ttp/test_response_schemas.py

@@ -42,10 +42,6 @@ def test_placeholder_golden_is_stable() -> None:
     )
 
 
-@pytest.mark.xfail(
-    strict=True,
-    reason="impl phase E.3.8: TTP router not yet contributing to OpenAPI",
-)
 def test_openapi_includes_ttp_paths() -> None:
     """Every documented TTP endpoint must appear in the live OpenAPI
     schema once the router lands. Pinned as a strict-xfail so the