feat(workflows): implement prepare_event so booking_full/booking_simple are enableable

The booking_full / booking_simple flows go prepare_event -> prepare_invoice,
but prepare_event was still a guard-stub, so enabling either flow returned
409 'uses actions that aren't implemented: prepare_event'.

prepare_event now calls convertToEvent({ hold: true }): convertToEvent already
creates the event as is_draft=true AND schedules its invoices, so this creates
those invoices on HOLD (scheduled_send_at NULL) and stashes their ids in
ctx.vars.preparedInvoiceIds. The downstream prepare_invoice already short-
circuits on a populated preparedInvoiceIds, so it ADOPTS the event's held
invoices instead of calling convertToInvoiceOnly again (which would both
double-create and throw ALREADY_CONVERTED_TO_EVENT). The review gate, the
wait-until-event-date, and send_document then issue those same invoices.
send_document(event)=publish is intentionally left a graceful skip — the
gallery is published manually after photos are uploaded, not auto-published
on an empty draft.

convertToEvent gains the same single-connection SQLite deadlock fixes as
convertToInvoiceOnly (getAppSetting reads through trx; logActivity moved after
commit) since prepare_event runs unattended, returns invoiceIds (incl. the
idempotent already-converted re-entry, which recovers them by event_id), and
removes prepare_event from the enable-guard list.

Adds a convertToEvent hold-mode test (draft event + held invoices + quote
linkage) and updates the enable-guard test to a still-stub action
(prepare_gallery). Full backend suite: 982 passed, 1 skipped.

backend/__tests__/integration/workflowRoutes.test.js

@@ -74,13 +74,13 @@ describe('admin workflows API', () => {
   test('refuses to enable a flow that uses an unimplemented action', async () => {
     const create = await request(app).post('/api/admin/workflows').set(auth(token)).send({
       name: 'Stub flow', trigger_type: 'quote.accepted', enabled: false,
-      nodes: [{ node_key: 't', type: 'trigger' }, { node_key: 'a', type: 'action', config: { action: 'prepare_event' } }],
+      nodes: [{ node_key: 't', type: 'trigger' }, { node_key: 'a', type: 'action', config: { action: 'prepare_gallery' } }],
       edges: [{ from_node: 't', to_node: 'a' }],
     });
     expect(create.status).toBe(201);
     const res = await request(app).patch(`/api/admin/workflows/${create.body.id}/enabled`).set(auth(token)).send({ enabled: true });
     expect(res.status).toBe(409);
-    expect(res.body.error).toMatch(/not.*implemented|prepare_event/i);
+    expect(res.body.error).toMatch(/not.*implemented|prepare_gallery/i);
   });
 
   test('allows enabling a flow using the now-implemented booking invoice actions', async () => {