Implement flexible entity relation system (Phases A–E)

Declarative relation registry via defrelation s-expressions with
cardinality enforcement (one-to-one, one-to-many, many-to-many),
registry-aware relate/unrelate/can-relate API endpoints, generic
container-nav fragment, and relation-driven UI components.

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

relations/app.py

@@ -3,7 +3,7 @@ import path_setup  # noqa: F401
 
 from shared.infrastructure.factory import create_base_app
 
-from bp import register_actions, register_data
+from bp import register_actions, register_data, register_fragments
 from services import register_domain_services
 
 
@@ -15,6 +15,7 @@ def create_app() -> "Quart":
 
     app.register_blueprint(register_actions())
     app.register_blueprint(register_data())
+    app.register_blueprint(register_fragments())
 
     return app