feat(customers): customer portal (#354) on top of feature-flags reorg

Implements the recurring-customer login surface from
the-luap/picpeak#354 plugged into the maintainer's
new feature-flag infrastructure (PR #443) instead of
a parallel toggle.

* New `customerPortal` feature flag (foundation flag for the
  not-yet-built calendar/quotes/bills/messaging customer
  surfaces). Defaults FALSE on fresh installs, TRUE on existing
  installs (events > 0) via migration 095 so live customer
  accounts don't disappear mid-deployment.
* Foundation schema: customer_accounts, customer_invitations,
  event_customer_assignments, customer_password_resets, plus
  RBAC permissions customers.view / .create / .delete granted
  to super_admin + admin system roles.
* Backend: /api/admin/customers (invite, list, search, assign,
  deactivate, reset password) + /api/customer/auth/* +
  /api/customer/* (login, dashboard, accept-invite, reset).
  Customer JWT bypass minted via
  /api/customer/events/:slug/access-token so existing gallery
  middleware stays untouched.
* Frontend: /customer/* route tree gated by RequireFeature flag
  customerPortal, with login / dashboard / accept-invite /
  reset pages and a customer-side sidebar layout.
  /admin/customers and /admin/customers/:id gated identically.
* Settings → Features grows a "Customers" section with a
  Customer portal card. The maintainer's Features tab stays the
  single source of truth — no parallel Advanced features tab.
* CustomerAccountPicker on event create/edit forms hides itself
  when the flag is off; backend ignores customer_account_ids in
  that case instead of erroring the whole event save.

Translations: en + de hand-translated. nl/pt/ru fall through to
en — flagged here as needing native review.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

backend/server.js

@@ -555,7 +555,6 @@ app.use('/api/gallery', require('./src/routes/galleryGuests'));
 app.use('/api/admin', adminRoutes);
 app.use('/api/admin/auth', adminAuthRoutes);
 app.use('/api/admin/system', require('./src/routes/adminSystem'));
-app.use('/api/admin/feature-flags', require('./src/routes/adminFeatureFlags'));
 app.use('/api/admin/backup', require('./src/routes/adminBackup'));
 app.use('/api/admin/database-backup', require('./src/routes/adminDatabaseBackup'));
 app.use('/api/admin/feedback', require('./src/routes/adminFeedback'));
@@ -568,6 +567,21 @@ app.use('/api/admin/photo-export', require('./src/routes/adminPhotoExport'));
 app.use('/api/admin/css-templates', require('./src/routes/adminCssTemplates'));
 app.use('/api/admin/events', require('./src/routes/adminEventRename'));
 app.use('/api/admin/users', require('./src/routes/adminUsers'));
+// Customer-portal feature gate (#354 follow-up). When the master
+// toggle in Settings → Advanced features is OFF, the customer surface
+// returns 410 Gone for end-user routes and 403 for admin-side
+// management routes. The gate fires before the route handler so we
+// don't pay for auth checks against a disabled feature.
+const {
+  requireCustomerPortalEnabled,
+  requireCustomerPortalEnabledAdmin,
+} = require('./src/middleware/requireCustomerPortal');
+
+app.use('/api/admin/customers', requireCustomerPortalEnabledAdmin, require('./src/routes/adminCustomers'));
+// Customer-side surface (#354). Strictly separate from /api/admin/* —
+// distinct token type, distinct cookie, distinct middleware.
+app.use('/api/customer/auth', requireCustomerPortalEnabled, require('./src/routes/customerAuth'));
+app.use('/api/customer', requireCustomerPortalEnabled, require('./src/routes/customer'));
 app.use('/api/admin/event-types', require('./src/routes/adminEventTypes'));
 app.use('/api/admin/api-tokens', require('./src/routes/adminApiTokens'));
 app.use('/api/admin/webhooks', require('./src/routes/adminWebhooks'));