debug: add /debug/routers endpoint to diagnose import failures

Crosswalk routes returning 404 on production. This adds a diagnostic
endpoint that reports which sub-routers failed to load and why.

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

backend-compliance/compliance/api/__init__.py

@@ -6,6 +6,8 @@ from .routes import router
 
 logger = logging.getLogger(__name__)
 
+_failed_routers: dict[str, str] = {}
+
 
 def _safe_import_router(module_name: str, attr: str = "router"):
     """Import a router module safely — log error but don't crash the whole app."""
@@ -14,6 +16,7 @@ def _safe_import_router(module_name: str, attr: str = "router"):
         return getattr(mod, attr)
     except Exception as e:
         logger.error("Failed to import %s: %s", module_name, e)
+        _failed_routers[module_name] = str(e)
         return None