feat: production routing support for subdomain and custom domain modes

Double-mount store routes at /store/* and /store/{store_code}/* so the
same handlers work in dev path-based, prod path-based, prod subdomain,
and prod custom-domain modes.  Wire StorePlatform.custom_subdomain into
StoreContextMiddleware for per-platform subdomain overrides.  Add admin
custom-domain management UI, fix stale /shop/ reset link, add
/merchants/ to reserved paths, and server-render window.STORE_CODE for
JS that previously parsed the URL.

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

middleware/store_context.py

@@ -185,6 +185,35 @@ class StoreContextManager:
         # Method 2 & 3: Subdomain or path-based lookup
         if "subdomain" in context:
             subdomain = context["subdomain"]
+
+            # 2a. Check StorePlatform.custom_subdomain (platform-specific override)
+            #     e.g. acme-rewards.rewardflow.lu → StorePlatform with custom_subdomain="acme-rewards"
+            platform = context.get("_platform")
+            if platform and context.get("detection_method") == "subdomain":
+                from app.modules.tenancy.models.store_platform import StorePlatform
+
+                store_platform = (
+                    db.query(StorePlatform)
+                    .filter(
+                        func.lower(StorePlatform.custom_subdomain) == subdomain.lower(),
+                        StorePlatform.platform_id == platform.id,
+                        StorePlatform.is_active.is_(True),
+                    )
+                    .first()
+                )
+                if store_platform:
+                    store = (
+                        db.query(Store)
+                        .filter(Store.id == store_platform.store_id, Store.is_active.is_(True))
+                        .first()
+                    )
+                    if store:
+                        logger.info(
+                            f"[OK] Store found via custom_subdomain: {subdomain} → {store.name} (platform={platform.code})"
+                        )
+                        return store
+
+            # 2b. Fallback to Store.subdomain (global default)
             store = (
                 db.query(Store)
                 .filter(func.lower(Store.subdomain) == subdomain.lower())
@@ -493,6 +522,12 @@ class StoreContextMiddleware(BaseHTTPMiddleware):
         store_context = StoreContextManager.detect_store_context(request)
 
         if store_context:
+            # Pass platform from middleware state so subdomain lookup can check
+            # StorePlatform.custom_subdomain for platform-specific overrides
+            platform = getattr(request.state, "platform", None)
+            if platform:
+                store_context["_platform"] = platform
+
             db_gen = get_db()
             db = next(db_gen)
             try: