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/bookingCutover.test.js

@@ -56,6 +56,26 @@ describe('booking cutover — draft invoices on hold', () => {
     expect(inv.scheduled_send_at == null).toBe(false); // normal convert → auto-send date set
   });
 
+  it('prepare_event path (convertToEvent hold) creates a DRAFT event with held invoices', async () => {
+    const quoteId = await acceptedQuote();
+    const res = await quoteService.convertToEvent(quoteId, adminId, { hold: true });
+    expect(res.eventId).toBeGreaterThanOrEqual(1);
+    expect(Array.isArray(res.invoiceIds)).toBe(true);
+    expect(res.invoiceIds.length).toBeGreaterThanOrEqual(1);
+
+    const ev = await db('events').where({ id: res.eventId }).first();
+    expect(ev.is_draft == true || ev.is_draft === 1).toBe(true); // created as a draft gallery
+
+    // Every invoice the event scheduled is held (no auto-send before the gate).
+    const invs = await db('invoices').whereIn('id', res.invoiceIds);
+    for (const inv of invs) expect(inv.scheduled_send_at == null).toBe(true);
+
+    // Quote is now linked to the event — convertToInvoiceOnly must NOT be called
+    // again for it (the flow's prepare_invoice adopts these ids instead).
+    const q = await db('quotes').where({ id: quoteId }).first();
+    expect(q.converted_event_id).toBe(res.eventId);
+  });
+
   it('prepare_contract path (createFromQuote) completes under SQLite — no in-trx deadlock', async () => {
     const contractService = require('../../src/services/contractService');
     const quoteId = await acceptedQuote();