Add Profile model + migration + own-profile endpoints (US-3)

Implements user profile data model and own-profile management endpoints:
- Profile and ProfileFavorites models in backend/app/models/profile.py
- Migration 0010_add_profile_tables.py (profiles, profile_favorites)
- Endpoints in backend/app/routers/profiles.py:
  - GET  /api/profile/me (own profile with default fallback)
  - PUT  /api/profile/me (upsert bio, avatar_asset_id, privacy settings)
  - GET  /api/profile/me/favorites (list favorite world IDs)
  - PUT  /api/profile/me/favorites (replace favorite world IDs)
- Fixes values_callable on Asset.asset_type ENUM in app/models/asset.py
- Added test_profile.py covering auth, validation, favorites, privacy

Independently re-verified: 19/19 new profile tests pass,
258/258 full backend regression suite passes with zero regressions.

backend/tests/conftest.py

@@ -36,6 +36,9 @@ from app.main import app
 
 TEST_TABLES = (
     "avatar_loadouts", "assets",
+    # Profile (app/models/profile.py) -- both FK users.id, profile_favorites
+    # additionally FKs worlds.id; list explicitly for clarity/consistency.
+    "profile_favorites", "profiles",
     # world_chunk_revisions has no FK to worlds/world_chunks (world.md §4.7 /
     # world_persistence_impl.md §4.3, deliberate -- deleting a chunk or world
     # must not lose the audit trail), so it won't be cascade-truncated via