Auto-mount fragment handlers: eliminate fragment blueprint boilerplate across all 8 services

Fragment read API is now fully declarative — every handler is a defhandler
s-expression dispatched through one shared auto_mount_fragment_handlers()
function. Replaces 8 near-identical blueprint files (~35 lines each) with
a single function call per service. Events Python handlers (container-cards,
account-page) extracted to a standalone module.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

blog/bp/__init__.py

@@ -2,6 +2,5 @@ from .blog.routes import register as register_blog_bp
 from .admin.routes import register as register_admin
 from .menu_items.routes import register as register_menu_items
 from .snippets.routes import register as register_snippets
-from .fragments import register_fragments
 from .data import register_data
 from .actions.routes import register as register_actions