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/main.py

@@ -6,7 +6,7 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from app.config import settings
-from app.routers import auth, users, worlds, assets, avatar, world_content, internal, presence, groups, social_friends
+from app.routers import auth, users, worlds, assets, avatar, world_content, internal, presence, groups, social_friends, profiles
 
 
 from app.services.game_servers import supervisor
@@ -47,6 +47,7 @@ app.include_router(internal.router, prefix="/api/internal", tags=["internal"])
 app.include_router(presence.router, prefix="/api/presence", tags=["presence"])
 app.include_router(groups.router, prefix="/api/groups", tags=["groups"])
 app.include_router(social_friends.router, prefix="/api/friends", tags=["friends"])
+app.include_router(profiles.router, prefix="/api/profile", tags=["profile"])
 
 
 @app.get("/api/health")