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>

shared/infrastructure/factory.py

@@ -30,6 +30,7 @@ from .jinja_setup import setup_jinja
 from .user_loader import load_current_user
 from shared.sexp.jinja_bridge import setup_sexp_bridge
 from shared.sexp.components import load_shared_components
+from shared.sexp.relations import load_relation_registry
 
 
 # Async init of config (runs once at import)
@@ -108,6 +109,7 @@ def create_base_app(
     setup_jinja(app)
     setup_sexp_bridge(app)
     load_shared_components()
+    load_relation_registry()
     errors(app)
 
     # Auto-register OAuth client blueprint for non-account apps