feat(events): duplicate-gallery action (#626)

Daniel asked for a way to re-use a good gallery configuration without
re-entering every setting. Two of his three suggested workflows are
covered by this PR; the third (per-event-type behaviour defaults) is
partially shipped already via event_types.theme_preset + theme_config
and is left as a follow-up if the duplicate workflow doesn't cover it.

Backend — POST /admin/events/:id/duplicate. Validates a new event_name
(required) + event_date (optional) + customer_name/email (optional);
copies branding (color_theme, css_template_id, header/hero/divider/anchor),
behaviour toggles (allow_downloads, watermark_*, allow_user_uploads,
require_password, etc.), photo_cap, welcome_message, default_photo_sort,
admin_email, and feedback settings + per-event photo categories. Mints a
fresh slug + share_token + random-placeholder password_hash (admin sets
the real one via the publish dialog shipped in #627). Recomputes
expires_at = new_event_date + (source.expires_at - source.event_date) so
the duplicate keeps the same active window; defaults to 30 days if either
source field was null. is_draft is always true.

Deliberately NOT carried over: photos, hero_photo_id, client_access
secrets, og_image_share opt-in, customer_phone, sent_at flags, archive
state, customer-account assignments.

Frontend — new DuplicateEventDialog (matches the PublishGalleryDialog
pattern), wired into the Actions card on EventDetailsPage. Visible in
both draft and live mode since admins typically duplicate from a
published gallery. On success the page navigates to the new draft so the
admin can finish customising + publish.

I18n: EN + DE entries for the dialog + button label. Backend logs an
event_duplicated activity with the source event id/name so the trail is
auditable.

Frontend service: eventsService.duplicateEvent(eventId, data).

frontend/src/services/events.service.ts

@@ -232,6 +232,22 @@ export const eventsService = {
     return response.data;
   },
 
+  // Duplicate an event (#626). Creates a new draft gallery that inherits the
+  // source event's branding + behaviour + feedback + categories. Photos are
+  // NOT carried over. The returned id/slug are the new draft event.
+  async duplicateEvent(
+    eventId: number,
+    data: {
+      event_name: string;
+      event_date?: string;
+      customer_name?: string;
+      customer_email?: string;
+    },
+  ): Promise<{ message: string; id: number; slug: string; is_draft: boolean }> {
+    const response = await api.post(`/admin/events/${eventId}/duplicate`, data);
+    return response.data;
+  },
+
   // Get admin preview token (uses existing admin session token)
   getPreviewToken(): string | null {
     const token = sessionStorage.getItem('admin_token') || localStorage.getItem('admin_token');