feat(footer): hideable legal links + socials + promo banner (#441 + #440)

Combined footer overhaul:

- Per-CMS-page show_in_footer toggle (#441) — admins can hide
  Impressum / Datenschutz from the gallery footer when an external
  privacy / imprint URL is enough.
- Five social-media URL fields in branding settings (#441) — Facebook,
  Instagram, WhatsApp, X/Twitter, YouTube. Empty string hides each
  icon individually; the row is omitted when none are set.
- Promotional banner slot above or below the gallery footer (#440) —
  global default authored as markdown in branding settings, plus a
  three-way per-event override on the Edit Event form
  (inherit / custom / off). Backend nulls promo_markdown automatically
  when mode != 'custom' so stale text never persists.

Sanitization: marked with gfm/breaks → DOMPurify with a tight
allowlist (no img, no tables, no inline html). Post-process forces
target=_blank rel="noopener noreferrer nofollow" on every link so
admin-set URLs can't tab-nap the gallery context.

i18n covers all six locales (en/de/nl/pt/ru/fr).

Targets the beta branch.

backend/migrations/core/089_footer_overhaul.js

@@ -0,0 +1,119 @@
+/**
+ * Migration 089: Footer overhaul (#441 + #440).
+ *
+ * Three concerns, all in the gallery footer area:
+ *
+ * 1. Per-CMS-page "Show in footer" toggle (#441 part a). Lets admins
+ *    hide legal-link entries (Impressum, Datenschutz, etc.) when the
+ *    target jurisdiction doesn't require them. Default TRUE so existing
+ *    installs see no change.
+ *
+ * 2. Social links in the footer (#441 part b). Five branding settings
+ *    for the canonical photographer-relevant networks: Facebook,
+ *    Instagram, WhatsApp, X/Twitter, YouTube. All optional strings.
+ *
+ * 3. Promotional markdown slot above/below the footer (#440). Global
+ *    default + per-event override with a three-way mode switch:
+ *      - inherit (default): use the global, render nothing if global empty
+ *      - custom:            render the per-event markdown
+ *      - off:               suppress entirely for this event regardless of global
+ *
+ *    Markdown only (no raw HTML) — the rendering pipeline is
+ *    `marked → DOMPurify` on the frontend so admins can format text
+ *    without opening an XSS surface.
+ */
+
+exports.up = async function(knex) {
+  console.log('Running migration: 089_footer_overhaul');
+
+  // 1. cms_pages.show_in_footer
+  const hasShowInFooter = await knex.schema.hasColumn('cms_pages', 'show_in_footer');
+  if (!hasShowInFooter) {
+    await knex.schema.alterTable('cms_pages', (table) => {
+      table.boolean('show_in_footer').notNullable().defaultTo(true);
+    });
+    console.log('  added cms_pages.show_in_footer (default true)');
+  } else {
+    console.log('  cms_pages.show_in_footer already exists, skipping');
+  }
+
+  // 2. events.promo_mode + events.promo_markdown
+  const hasPromoMode = await knex.schema.hasColumn('events', 'promo_mode');
+  if (!hasPromoMode) {
+    await knex.schema.alterTable('events', (table) => {
+      table.string('promo_mode', 16).notNullable().defaultTo('inherit');
+    });
+    console.log('  added events.promo_mode (default "inherit")');
+  } else {
+    console.log('  events.promo_mode already exists, skipping');
+  }
+
+  const hasPromoMarkdown = await knex.schema.hasColumn('events', 'promo_markdown');
+  if (!hasPromoMarkdown) {
+    await knex.schema.alterTable('events', (table) => {
+      table.text('promo_markdown').nullable();
+    });
+    console.log('  added events.promo_markdown (nullable text)');
+  } else {
+    console.log('  events.promo_markdown already exists, skipping');
+  }
+
+  // 3. New branding settings rows. Use the same `branding_<name>`
+  //    convention as the existing 21 branding rows (verified via
+  //    SELECT setting_key FROM app_settings WHERE setting_key LIKE 'branding_%').
+  const newSettings = [
+    { setting_key: 'branding_facebook_url',   setting_value: JSON.stringify(''), setting_type: 'branding' },
+    { setting_key: 'branding_instagram_url',  setting_value: JSON.stringify(''), setting_type: 'branding' },
+    { setting_key: 'branding_whatsapp_url',   setting_value: JSON.stringify(''), setting_type: 'branding' },
+    { setting_key: 'branding_twitter_url',    setting_value: JSON.stringify(''), setting_type: 'branding' },
+    { setting_key: 'branding_youtube_url',    setting_value: JSON.stringify(''), setting_type: 'branding' },
+    { setting_key: 'branding_promo_markdown', setting_value: JSON.stringify(''), setting_type: 'branding' },
+    // 'above_footer' | 'below_footer' (string instead of enum so we can
+    // expand without a schema change later).
+    { setting_key: 'branding_promo_position', setting_value: JSON.stringify('above_footer'), setting_type: 'branding' },
+  ];
+
+  for (const setting of newSettings) {
+    const exists = await knex('app_settings').where('setting_key', setting.setting_key).first();
+    if (!exists) {
+      await knex('app_settings').insert({ ...setting, updated_at: knex.fn.now() });
+    }
+  }
+  console.log(`  ensured ${newSettings.length} branding rows`);
+
+  console.log('Migration 089_footer_overhaul completed');
+};
+
+exports.down = async function(knex) {
+  console.log('Rollback: 089_footer_overhaul');
+
+  if (await knex.schema.hasColumn('cms_pages', 'show_in_footer')) {
+    await knex.schema.alterTable('cms_pages', (table) => {
+      table.dropColumn('show_in_footer');
+    });
+  }
+
+  if (await knex.schema.hasColumn('events', 'promo_mode')) {
+    await knex.schema.alterTable('events', (table) => {
+      table.dropColumn('promo_mode');
+    });
+  }
+
+  if (await knex.schema.hasColumn('events', 'promo_markdown')) {
+    await knex.schema.alterTable('events', (table) => {
+      table.dropColumn('promo_markdown');
+    });
+  }
+
+  await knex('app_settings')
+    .whereIn('setting_key', [
+      'branding_facebook_url',
+      'branding_instagram_url',
+      'branding_whatsapp_url',
+      'branding_twitter_url',
+      'branding_youtube_url',
+      'branding_promo_markdown',
+      'branding_promo_position',
+    ])
+    .del();
+};