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/app/models/asset.py

@@ -23,7 +23,12 @@ class Asset(Base):
     name = Column(String(255), nullable=False)
     description = Column(Text, nullable=True)
     asset_type = Column(
-        ENUM(AssetType, name="asset_type", create_type=False),
+        ENUM(
+            AssetType,
+            name="asset_type",
+            create_type=False,
+            values_callable=lambda enum_cls: [e.value for e in enum_cls],
+        ),
         nullable=False,
     )
     file_path = Column(String(512), nullable=False)