feat(workflows): implement remaining stub actions (prepare_quote, prepare_gallery, reserve_date)

These were the last guard-stubbed actions — offered in the builder palette but
refused on enable. Now all three are real, backed by existing converters:

- prepare_gallery: alias of prepare_event (a gallery IS an event in picpeak).
- reserve_date: convertToEvent({ skipInvoices: true }) — a pure draft date hold
  with no money documents (new skipInvoices option on convertToEvent).
- prepare_quote: createQuote (customer entity) or duplicateQuote (quote entity),
  producing a status='draft' quote; idempotent via ctx.vars.preparedQuoteId.

With no stubs left, the enable-guard switches from a hardcoded DOCUMENT_ACTIONS
list to a registry lookup: an action node whose config.action has no registered
handler is unimplementable. This can't drift from what the engine can run and
also catches typo'd/future actions. (Fixes the enable-route node mapping to
carry node.type so the action-node filter matches.)

Extends the single-connection SQLite in-trx deadlock fixes to the quote-create
path (prepare_quote runs unattended): nextQuoteNumber reads getAppSetting
through trx, createQuote logs via trx and hoists its hasColumnCached schema-drift
checks before the transaction.

Adds tests for reserve_date (no invoices), prepare_quote (draft, no deadlock),
and registry coverage; retargets the enable-guard refusal test at a genuinely
unregistered action. Full backend suite: 985 passed, 1 skipped.

backend/__tests__/integration/workflowRoutes.test.js

@@ -71,16 +71,16 @@ describe('admin workflows API', () => {
     expect(res.body.error).toMatch(/unknown node type/i);
   });
 
-  test('refuses to enable a flow that uses an unimplemented action', async () => {
+  test('refuses to enable a flow that uses an unregistered 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_gallery' } }],
+      nodes: [{ node_key: 't', type: 'trigger' }, { node_key: 'a', type: 'action', config: { action: 'totally_not_a_real_action' } }],
       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_gallery/i);
+    expect(res.body.error).toMatch(/not.*implemented|totally_not_a_real_action/i);
   });
 
   test('allows enabling a flow using the now-implemented booking invoice actions', async () => {