feat(events): "Create invoice" action on the event detail page

Add a bills-gated button that opens the bill editor pre-filled with the
event (eventId FK + name/date snapshot) and the linked customer (when
exactly one). BillEditorPage gains eventId state + query-param prefill +
sends eventId on create; backend validates eventId (already forwarded +
persisted). Reuses the editor — no empty drafts. Does not auto-pull hours.

backend/src/routes/adminInvoices.js

@@ -240,6 +240,9 @@ const INVOICE_BODY_VALIDATORS = [
   body('skontoDisabled').optional().isBoolean(),
   // Inline event snapshot (migration 123). Mirrors quotes — kept
   // optional because standalone invoices may not have an event yet.
+  // eventId links the invoice to a gallery event (FK) when created from
+  // the event detail page; persisted by createInvoice (event_id).
+  body('eventId').optional({ values: 'falsy' }).isInt({ min: 1 }),
   body('eventName').optional({ values: 'falsy' }).isString().isLength({ max: 255 }),
   body('eventDate').optional({ values: 'falsy' }).isISO8601(),
   body('eventTimeStart').optional({ values: 'falsy' }).isString().isLength({ max: 8 }),

frontend/src/i18n/locales/de.json

@@ -840,6 +840,7 @@
     "copyLink": "Link kopieren",
     "linkCopied": "Link kopiert!",
     "viewGallery": "Galerie ansehen",
+    "createInvoice": "Rechnung erstellen",
     "uploadPhotos": "Fotos hochladen",
     "archiveEvent": "Veranstaltung archivieren",
     "archiveConfirm": "Sind Sie sicher, dass Sie diese Veranstaltung archivieren möchten? Diese Aktion kann nicht rückgängig gemacht werden.",

frontend/src/i18n/locales/en.json

@@ -399,6 +399,7 @@
     "copyLink": "Copy Link",
     "linkCopied": "Link copied!",
     "viewGallery": "View Gallery",
+    "createInvoice": "Create invoice",
     "uploadPhotos": "Upload Photos",
     "archiveEvent": "Archive Event",
     "archiveConfirm": "Are you sure you want to archive this event? This action cannot be undone.",

frontend/src/pages/admin/EventDetailsPage.tsx

@@ -18,6 +18,7 @@ import {
   Key,
   Mail,
   MessageSquare,
+  Receipt,
   Lock,
   Eye,
   EyeOff,
@@ -980,6 +981,26 @@ export const EventDetailsPage: React.FC = () => {
                         {t('feedback.manage', 'Manage Feedback')}
                       </Button>
                     )}
+                    {/* Create a draft invoice for this event — pre-fills the
+                        bill editor with the event snapshot + (when exactly
+                        one is linked) the customer. Gated on the bills flag. */}
+                    {flags.bills && (
+                      <Button
+                        variant="outline"
+                        size="sm"
+                        leftIcon={<Receipt className="w-4 h-4" />}
+                        onClick={() => {
+                          const accts = ((event as { customer_accounts?: Array<{ id: number }> }).customer_accounts) || [];
+                          const params = new URLSearchParams({ eventId: String(event.id) });
+                          if (event.event_name) params.set('eventName', event.event_name);
+                          if (event.event_date) params.set('eventDate', String(event.event_date).slice(0, 10));
+                          if (accts.length === 1) params.set('customerAccountId', String(accts[0].id));
+                          navigate(`/admin/clients/bills/new?${params.toString()}`);
+                        }}
+                      >
+                        {t('events.createInvoice', 'Create invoice')}
+                      </Button>
+                    )}
                   </>
                 )}
               </>

frontend/src/pages/admin/bills/BillEditorPage.tsx

@@ -83,6 +83,7 @@ export const BillEditorPage: React.FC = () => {
   // event section — admin can type a free-text label without needing
   // an actual events row, and it carries through to the customer
   // portal + tax report + email templates.
+  const [eventId, setEventId] = useState<number | null>(null);
   const [eventName, setEventName] = useState('');
   const [eventDate, setEventDate] = useState('');
   const [eventTimeStart, setEventTimeStart] = useState('');
@@ -143,6 +144,7 @@ export const BillEditorPage: React.FC = () => {
       setPaymentTimingTemplateId(inv.paymentTimingTemplateId ?? null);
       setBusinessBankAccountId(inv.businessBankAccountId ?? null);
       setSkontoDisabled(Boolean(inv.skontoDisabled));
+      setEventId(inv.eventId ?? null);
       setEventName(inv.eventName || '');
       setEventDate(inv.eventDate || '');
       setEventTimeStart(inv.eventTimeStart || '');
@@ -216,6 +218,23 @@ export const BillEditorPage: React.FC = () => {
     })();
   }, [isEdit, searchParams, customerId]);
 
+  // Pre-fill the event link + snapshot when opened from an event's
+  // "Create invoice" button (/admin/clients/bills/new?eventId=&eventName=&eventDate=).
+  const didPrefillEventRef = useRef(false);
+  useEffect(() => {
+    if (isEdit) return;
+    if (didPrefillEventRef.current) return;
+    const eidRaw = searchParams.get('eventId');
+    const eid = eidRaw ? parseInt(eidRaw, 10) : NaN;
+    const en = searchParams.get('eventName');
+    const ed = searchParams.get('eventDate');
+    if (!(Number.isFinite(eid) && eid > 0) && !en && !ed) return;
+    didPrefillEventRef.current = true;
+    if (Number.isFinite(eid) && eid > 0) setEventId(eid);
+    if (en) setEventName((prev) => prev || en);
+    if (ed) setEventDate((prev) => prev || ed);
+  }, [isEdit, searchParams]);
+
   // Pre-fill from a fully-signed contract when the editor is opened
   // via `?fromContractId=<id>` (the "New invoice" link on
   // ContractDetailPage's header, used after the contract has been
@@ -384,6 +403,7 @@ export const BillEditorPage: React.FC = () => {
     // so the backend can distinguish "not provided" from a deliberate
     // clear (which the route's `optional({ values: 'falsy' })` already
     // treats identically — falsy values bypass validation entirely).
+    eventId: eventId ?? undefined,
     eventName: eventName || undefined,
     eventDate: eventDate || undefined,
     eventTimeStart: eventTimeStart || undefined,