fix(security): reject array values for every field on the event update

Replaces the six per-field .not().isArray() guards from the previous
commit. Those were too narrow, and arbitrarily so.

PUT /:id spreads req.body into `updates` (crud.js:1631) and passes it to
.update() (:1990) with only targeted deletes in between — there is no
column allow-list. express-validator applies isInt/isIn/isBoolean
element-wise to arrays, so a single-element array satisfies its field
validator and survives the whole way to the column. That is true of all
44 validated fields, not of the protection block I happened to be
looking at; seven of them also run through formatBoolean, where [false]
reads as true.

So the guard belongs where the body is spread, not on chosen fields.
`customer_account_ids` is the only field legitimately an array — it has
an isArray() validator and its own element rules — and it is deleted
from `updates` before the write, so exempting it costs nothing.

Tested across the protection fields and two outside that block, plus the
customer_account_ids exemption. With the guard's condition disabled,
exactly those six array cases fail and the other 15 in the suite pass.

Refs #1296

backend/__tests__/routes/adminEvents.smoke.test.js

@@ -214,6 +214,44 @@ describe('admin events CRUD endpoints (smoke)', () => {
       expect(row.welcome_message).toBe('Hello guests');
     });
 
+    // #1296 — express-validator runs isInt/isIn/isBoolean element-wise on
+    // arrays, so a single-element array satisfies its field validator and
+    // survives into `updates`, which is spread into .update() with no column
+    // allow-list. That put an array into a scalar column (a PG insert error),
+    // and formatBoolean([false]) read as true. Guarded for every field, not
+    // just the ones that prompted it.
+    it.each([
+      ['image_quality', [72]],
+      ['protection_level', ['basic']],
+      ['use_canvas_rendering', [false]],
+      ['fragmentation_level', [3]],
+      // Not a protection field: the guard is not scoped to that block.
+      ['event_name', ['Arrayed']],
+      ['allow_downloads', [false]],
+    ])('400s on an array value for %s', async (field, value) => {
+      const id = await insertEvent(db, adminId, { event_name: 'Unchanged' });
+      const res = await auth(request(app).put(`/api/admin/events/${id}`))
+        .send({ [field]: value });
+
+      expect(res.status).toBe(400);
+      expect(res.body.error).toMatch(field);
+      // And nothing was written.
+      const row = await db('events').where({ id }).first();
+      expect(row.event_name).toBe('Unchanged');
+    });
+
+    it('still accepts customer_account_ids, the one field that is an array', async () => {
+      const id = await insertEvent(db, adminId, { event_name: 'Keep' });
+      const res = await auth(request(app).put(`/api/admin/events/${id}`)).send({
+        event_name: 'Renamed',
+        customer_account_ids: [],
+      });
+
+      expect(res.status).toBe(200);
+      const row = await db('events').where({ id }).first();
+      expect(row.event_name).toBe('Renamed');
+    });
+
     it('404s when updating a missing event', async () => {
       const res = await auth(request(app).put('/api/admin/events/999999')).send({
         event_name: 'Ghost',