refactor(P6): standardize route variable naming to router

All route files (admin.py, store.py) now export `router` instead of
`admin_router`/`store_router`. Consumer code (definition.py, __init__.py)
imports as `router as admin_router` where distinction is needed.
ModuleDefinition fields remain admin_router/store_router.

64 files changed across all modules. Architecture rules, docs, and
migration plan updated. Added noqa:API001 support to validator for
pre-existing raw dict endpoints now visible with standardized router name.
All 1114 tests pass.

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

app/api/deps.py

@@ -474,11 +474,11 @@ def require_module_access(module_code: str, frontend_type: FrontendType):
     tied to a specific menu item.
 
     Usage:
-        admin_router = APIRouter(
+        router = APIRouter(
             dependencies=[Depends(require_module_access("messaging", FrontendType.ADMIN))]
         )
 
-        store_router = APIRouter(
+        router = APIRouter(
             dependencies=[Depends(require_module_access("billing", FrontendType.STORE))]
         )