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/app.py

@@ -16,7 +16,6 @@ from bp import (
     register_admin,
     register_menu_items,
     register_snippets,
-    register_fragments,
     register_data,
     register_actions,
 )
@@ -108,7 +107,9 @@ def create_app() -> "Quart":
     app.register_blueprint(register_admin("/settings"))
     app.register_blueprint(register_menu_items())
     app.register_blueprint(register_snippets())
-    app.register_blueprint(register_fragments())
+    from shared.sx.handlers import auto_mount_fragment_handlers
+    auto_mount_fragment_handlers(app, "blog")
+
     app.register_blueprint(register_data())
     app.register_blueprint(register_actions())