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>

app/modules/billing/routes/pages/store.py

@@ -7,11 +7,15 @@ Store pages for billing management:
 - Invoices
 """
 
-from fastapi import APIRouter, Depends, Path, Request
+from fastapi import APIRouter, Depends, Request
 from fastapi.responses import HTMLResponse
 from sqlalchemy.orm import Session
 
-from app.api.deps import get_current_store_from_cookie_or_header, get_db
+from app.api.deps import (
+    get_current_store_from_cookie_or_header,
+    get_db,
+    get_resolved_store_code,
+)
 from app.modules.core.utils.page_context import get_store_context
 from app.modules.tenancy.models import User
 from app.templates_config import templates
@@ -25,11 +29,11 @@ router = APIRouter()
 
 
 @router.get(
-    "/{store_code}/billing", response_class=HTMLResponse, include_in_schema=False
+    "/billing", response_class=HTMLResponse, include_in_schema=False
 )
 async def store_billing_page(
     request: Request,
-    store_code: str = Path(..., description="Store code"),
+    store_code: str = Depends(get_resolved_store_code),
     current_user: User = Depends(get_current_store_from_cookie_or_header),
     db: Session = Depends(get_db),
 ):
@@ -44,11 +48,11 @@ async def store_billing_page(
 
 
 @router.get(
-    "/{store_code}/invoices", response_class=HTMLResponse, include_in_schema=False
+    "/invoices", response_class=HTMLResponse, include_in_schema=False
 )
 async def store_invoices_page(
     request: Request,
-    store_code: str = Path(..., description="Store code"),
+    store_code: str = Depends(get_resolved_store_code),
     current_user: User = Depends(get_current_store_from_cookie_or_header),
     db: Session = Depends(get_db),
 ):

app/modules/billing/static/shared/js/feature-store.js

@@ -178,10 +178,11 @@
         },
 
         /**
-         * Get store code from URL
+         * Get store code from server-rendered value or URL fallback
          * @returns {string|null}
          */
         getStoreCode() {
+            if (window.STORE_CODE) return window.STORE_CODE;
             const path = window.location.pathname;
             const segments = path.split('/').filter(Boolean);
             // Direct: /store/{code}/...

app/modules/billing/static/shared/js/upgrade-prompts.js

@@ -139,9 +139,10 @@
         },
 
         /**
-         * Get store code from URL
+         * Get store code from server-rendered value or URL fallback
          */
         getStoreCode() {
+            if (window.STORE_CODE) return window.STORE_CODE;
             const path = window.location.pathname;
             const segments = path.split('/').filter(Boolean);
             if (segments[0] === 'store' && segments[1]) {