Add public/private visibility for trees

- Add is_public field to Tree model (private by default)
- Update access control: users see default trees, public trees, or their own
- Update all tree endpoints (list, search, get, categories) with new visibility logic
- Default/system trees are automatically marked as public
- Add migration 004 to add is_public column and update existing defaults
- Fix pydantic settings to ignore extra env vars (DATABASE_URL_SYNC)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/core/config.py

@@ -52,6 +52,7 @@ class Settings(BaseSettings):
     class Config:
         env_file = ".env"
         case_sensitive = True
+        extra = "ignore"  # Ignore extra env vars like DATABASE_URL_SYNC
 
 
 settings = Settings()