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/i18n/locales/de.json

@@ -1027,6 +1027,22 @@
       "passwordHelp": "Gib das bei der Erstellung gesetzte Passwort erneut ein (oder wähle ein neues). Die E-Mail enthält genau diesen Text; das Backend hasht es erneut, sodass die Galerie-Anmeldung weiterhin funktioniert.",
       "errorMinLength": "Das Passwort muss mindestens 6 Zeichen lang sein."
     },
+    "duplicateEvent": "Galerie duplizieren",
+    "duplicateDialog": {
+      "title": "Galerie duplizieren",
+      "description": "Erstellt eine neue Entwurfsgalerie, die Branding, Verhalten, Feedback und Kategorien aus \"{{sourceEventName}}\" übernimmt. Fotos, Passwort und Share-Tokens werden NICHT übernommen.",
+      "eventNameLabel": "Neuer Veranstaltungsname *",
+      "eventNamePlaceholder": "z. B. Hochzeit Müller 2026",
+      "eventDateLabel": "Veranstaltungsdatum",
+      "eventDateHelp": "Leer lassen, um eine zufällige Endung in der Galerie-URL zu verwenden. Das Ablaufdatum wird aus diesem Datum zuzüglich des Ablauffensters der Quellgalerie berechnet.",
+      "customerNameLabel": "Kundenname",
+      "customerNamePlaceholder": "Optional – kann später ausgefüllt werden",
+      "customerEmailLabel": "Kunden-E-Mail",
+      "customerEmailPlaceholder": "Optional",
+      "confirm": "Duplikat erstellen",
+      "errorNameRequired": "Veranstaltungsname ist erforderlich.",
+      "successToast": "Galerie dupliziert."
+    },
     "draftBanner": "Diese Galerie befindet sich im Entwurfsmodus. Laden Sie Ihre Fotos hoch und veröffentlichen Sie, wenn Sie bereit sind.",
     "subtitle": "Verwalten Sie Ihre Fotogalerien und Veranstaltungen",
     "failedToLoadEvents": "Veranstaltungen konnten nicht geladen werden",

frontend/src/i18n/locales/en.json

@@ -574,6 +574,22 @@
       "passwordHelp": "Re-type the password set at creation (or pick a new one). The email includes this exact text; the backend re-hashes it so the gallery login still works.",
       "errorMinLength": "Password must be at least 6 characters long."
     },
+    "duplicateEvent": "Duplicate gallery",
+    "duplicateDialog": {
+      "title": "Duplicate gallery",
+      "description": "Creates a new draft gallery that inherits the branding, behaviour, feedback, and category configuration from \"{{sourceEventName}}\". Photos, password, and share tokens are NOT carried over.",
+      "eventNameLabel": "New event name *",
+      "eventNamePlaceholder": "e.g. Müller Wedding 2026",
+      "eventDateLabel": "Event date",
+      "eventDateHelp": "Leave blank to use a random suffix in the gallery URL. Expiration is recomputed from this date plus the source gallery's expiration window.",
+      "customerNameLabel": "Customer name",
+      "customerNamePlaceholder": "Optional — fill in later if unknown",
+      "customerEmailLabel": "Customer email",
+      "customerEmailPlaceholder": "Optional",
+      "confirm": "Create duplicate",
+      "errorNameRequired": "Event name is required.",
+      "successToast": "Gallery duplicated."
+    },
     "draftBanner": "This gallery is in draft mode. Upload your photos, then publish when ready.",
     "subtitle": "Manage your photo galleries and events",
     "failedToLoadEvents": "Failed to load events",