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({