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>

federation/app.py

@@ -12,7 +12,6 @@ from shared.services.registry import services
 from bp import (
     register_identity_bp,
     register_social_bp,
-    register_fragments,
 )
 
 
@@ -99,7 +98,8 @@ def create_app() -> "Quart":
     from shared.sx.pages import auto_mount_pages
     auto_mount_pages(app, "federation")
 
-    app.register_blueprint(register_fragments())
+    from shared.sx.handlers import auto_mount_fragment_handlers
+    auto_mount_fragment_handlers(app, "federation")
 
     # --- home page ---
     @app.get("/")