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/user.py

@@ -35,6 +35,14 @@ class User(Base):
         cascade="all, delete-orphan",
     )
 
+    # Profile (app/models/profile.py): bio/avatar picture/privacy + favorites.
+    profile = relationship(
+        "Profile",
+        back_populates="user",
+        uselist=False,
+        cascade="all, delete-orphan",
+    )
+
     # Social graph (docs/planning/social.md US-1; app/models/social.py)
     sent_friend_requests = relationship(
         "Friendship",