refactor: migrate Feature to billing module and split ProductMedia to catalog

- Move Feature model from models/database/ to app/modules/billing/models/
  (tightly coupled to SubscriptionTier for tier-based access control)
- Move ProductMedia from models/database/media.py to app/modules/catalog/models/
  (product-specific media associations belong with catalog)
- Keep MediaFile as CORE in models/database/media.py (cross-cutting file storage)
- Convert legacy feature.py to re-export for backwards compatibility
- Update all imports to use canonical module locations

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

app/modules/cms/models/__init__.py

@@ -6,16 +6,15 @@ This is the canonical location for CMS models. Module models are automatically
 discovered and registered with SQLAlchemy's Base.metadata at startup.
 
 Usage:
-    from app.modules.cms.models import ContentPage, MediaFile, ProductMedia
+    from app.modules.cms.models import ContentPage
+
+For media models:
+    from models.database.media import MediaFile  # Core media file storage
+    from app.modules.catalog.models import ProductMedia  # Product-media associations
 """
 
 from app.modules.cms.models.content_page import ContentPage
 
-# Media models remain in core for now (used by multiple modules)
-from models.database.media import MediaFile, ProductMedia
-
 __all__ = [
     "ContentPage",
-    "MediaFile",
-    "ProductMedia",
 ]