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>

relations/app.py

@@ -4,7 +4,7 @@ import sx.sx_components as sx_components  # noqa: F401  # ensure Hypercorn --rel
 
 from shared.infrastructure.factory import create_base_app
 
-from bp import register_actions, register_data, register_fragments
+from bp import register_actions, register_data
 from services import register_domain_services
 
 
@@ -16,7 +16,9 @@ def create_app() -> "Quart":
 
     app.register_blueprint(register_actions())
     app.register_blueprint(register_data())
-    app.register_blueprint(register_fragments())
+
+    from shared.sx.handlers import auto_mount_fragment_handlers
+    auto_mount_fragment_handlers(app, "relations")
 
     return app