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>

middleware/storefront_access.py

@@ -83,6 +83,11 @@ def _is_static_request(path: str) -> bool:
     return "favicon.ico" in lower
 
 
+def _looks_like_storefront(path: str) -> bool:
+    """Return True if path belongs to the storefront surface."""
+    return path.startswith(("/storefront/", "/api/v1/storefront/"))
+
+
 class StorefrontAccessMiddleware(BaseHTTPMiddleware):
     """
     Gate storefront requests behind an active subscription.
@@ -94,6 +99,25 @@ class StorefrontAccessMiddleware(BaseHTTPMiddleware):
     async def dispatch(self, request: Request, call_next) -> Response:
         frontend_type = getattr(request.state, "frontend_type", None)
 
+        # Safety net: if frontend_type is None (upstream middleware failed) but
+        # the path looks like a storefront path, block instead of letting it
+        # through — a None frontend_type must never bypass the gate.
+        if frontend_type is None and _looks_like_storefront(request.url.path):
+            logger.error(
+                "[STOREFRONT_ACCESS] frontend_type is None on storefront path "
+                f"'{request.url.path}' — blocking (fail-closed). "
+                "Check middleware chain ordering."
+            )
+            if request.url.path.startswith("/api/"):
+                return JSONResponse(
+                    status_code=403,
+                    content={
+                        "error": "storefront_not_available",
+                        "reason": "middleware_misconfigured",
+                    },
+                )
+            return self._render_unavailable(request, "not_found")
+
         # Only gate storefront requests
         if frontend_type != FrontendType.STOREFRONT:
             return await call_next(request)