feat: optional customer phone field gated by global toggle (#322)

Adds a `customer_phone` column on events plus an `event_phone_field_enabled`
admin setting (default off) that surfaces the input in the create-event
and event-detail forms. Designed for downstream automation tooling — once
exposed via the upcoming public API, n8n / similar can pick it up to
deliver gallery links over WhatsApp, SMS, etc.

- Migration 080 adds the column + seeds the setting as false. Existing
  deployments see no UI change unless the admin opts in via
  Settings → Events.
- Backend strips the field server-side when the toggle is off (defence
  in depth against form bypass).
- Frontend renders the input only when the public-settings flag is true;
  always optional even then.
- publicSettings + EventSettings types extended; CreateEventPage and
  EventDetailsPage wired to read the toggle and submit the value.

frontend/src/features/settings/hooks/useSettingsState.ts

@@ -52,6 +52,7 @@ export interface EventSettings {
   event_require_expiration: boolean;
   event_default_require_password: boolean;
   gallery_show_filter_bar: boolean;
+  event_phone_field_enabled: boolean;
 }
 
 export interface SeoSettings {
@@ -127,7 +128,8 @@ export function useSettingsState() {
     event_require_event_date: true,
     event_require_expiration: true,
     event_default_require_password: true,
-    gallery_show_filter_bar: true
+    gallery_show_filter_bar: true,
+    event_phone_field_enabled: false
   });
 
   // SEO settings state
@@ -212,7 +214,8 @@ export function useSettingsState() {
         event_require_event_date: toBoolean(settings.event_require_event_date, true),
         event_require_expiration: toBoolean(settings.event_require_expiration, true),
         event_default_require_password: toBoolean(settings.event_default_require_password, true),
-        gallery_show_filter_bar: toBoolean(settings.gallery_show_filter_bar, true)
+        gallery_show_filter_bar: toBoolean(settings.gallery_show_filter_bar, true),
+        event_phone_field_enabled: toBoolean(settings.event_phone_field_enabled, false)
       });
 
       setSeoSettings({

frontend/src/features/settings/tabs/EventsTab.tsx

@@ -187,6 +187,25 @@ export const EventsTab: React.FC<EventsTabProps> = ({
               </div>
             </label>
           </div>
+
+          <div>
+            <label className="flex items-start gap-3">
+              <input
+                type="checkbox"
+                checked={eventSettings.event_phone_field_enabled}
+                onChange={(e) => setEventSettings(prev => ({ ...prev, event_phone_field_enabled: e.target.checked }))}
+                className="mt-1 w-4 h-4 text-primary-600 rounded focus:ring-primary-500"
+              />
+              <div>
+                <span className="text-sm font-medium text-neutral-700 dark:text-neutral-300">
+                  {t('settings.events.enablePhoneField', 'Enable phone number field')}
+                </span>
+                <p className="text-xs text-neutral-500 dark:text-neutral-400 mt-1">
+                  {t('settings.events.enablePhoneFieldHelp', 'Adds an optional phone number input to the event form. Useful for downstream automations like WhatsApp delivery via n8n. Always optional even when enabled.')}
+                </p>
+              </div>
+            </label>
+          </div>
         </div>
 
         <div className="mt-6">