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.

backend/migrations/core/080_add_customer_phone.js

@@ -0,0 +1,33 @@
+const { addColumnIfNotExists } = require('../helpers');
+
+/**
+ * #322 — optional phone-number field on events. Off by default; surfaced
+ * only when the global `event_phone_field_enabled` app setting is true,
+ * so existing deployments see no UI change unless the admin opts in.
+ */
+exports.up = async function up(knex) {
+  await addColumnIfNotExists(knex, 'events', 'customer_phone', (table) => {
+    table.string('customer_phone', 32).nullable();
+  });
+
+  // Seed the global enable flag (default false).
+  const exists = await knex('app_settings')
+    .where('setting_key', 'event_phone_field_enabled')
+    .first();
+  if (!exists) {
+    await knex('app_settings').insert({
+      setting_key: 'event_phone_field_enabled',
+      setting_value: JSON.stringify(false),
+      setting_type: 'boolean'
+    });
+  }
+};
+
+exports.down = async function down(knex) {
+  if (await knex.schema.hasColumn('events', 'customer_phone')) {
+    await knex.schema.alterTable('events', (table) => {
+      table.dropColumn('customer_phone');
+    });
+  }
+  await knex('app_settings').where('setting_key', 'event_phone_field_enabled').delete();
+};