feat(middleware): harden routing with fail-closed policy, custom subdomain management, and perf fixes

- Fix IPv6 host parsing with _strip_port() utility
- Remove dangerous StorePlatform→Store.subdomain silent fallback
- Close storefront gate bypass when frontend_type is None
- Add custom subdomain management UI and API for stores
- Add domain health diagnostic tool
- Convert db.add() in loops to db.add_all() (24 PERF-006 fixes)
- Add tests for all new functionality (18 subdomain service tests)
- Add .github templates for validator compliance

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

app/modules/tenancy/services/merchant_store_service.py

@@ -231,15 +231,17 @@ class MerchantStoreService:
 
             # Assign to platforms if provided
             platform_ids = store_data.get("platform_ids", [])
+            store_platforms = []
             for pid in platform_ids:
                 platform = db.query(Platform).filter(Platform.id == pid).first()
                 if platform:
-                    sp = StorePlatform(
+                    store_platforms.append(StorePlatform(
                         store_id=store.id,
                         platform_id=pid,
                         is_active=True,
-                    )
-                    db.add(sp)
+                    ))
+            if store_platforms:
+                db.add_all(store_platforms)
 
             db.flush()
             db.refresh(store)