fix(events): accept hero_logo_visible: null on create/update (#822)

hero_logo_visible is nullable — null means "inherit the global
branding_logo_display_hero toggle" (#756, migration 152). But the create and
update validators used `.optional()` without `{ nullable: true }`, which only
skips `undefined`; an explicit `null` still ran `.isBoolean()` and failed with
HTTP 400 "Invalid value". Saving an event with `hero_logo_visible: null` (the
inherit state the frontend sends) was rejected on v3.45.2.

- Both routes: `body('hero_logo_visible').optional({ nullable: true }).isBoolean()`,
  matching the already-correct `hero_logo_size` rule next to it.
- Create handler: guard on `!= null` instead of `!== undefined` so an explicit
  null stores NULL (inherit) rather than being coerced to 0/false by
  formatBoolean on SQLite. The update handler already did `=== null ? null`.

Left hero_logo_position on plain `.optional()` on purpose: its column is NOT
NULL (no inherit migration) and its handler always resolves to a concrete value
via `|| brandingDefaults`, so null is genuinely invalid there — allowing it
would trade the 400 for a 500.

Adds smoke tests: PUT accepts hero_logo_visible: null and stores NULL; a
non-boolean value is still rejected.

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

@@ -180,6 +180,27 @@ describe('admin events CRUD endpoints (smoke)', () => {
       });
       expect(res.status).toBe(404);
     });
+
+    // #822 — hero_logo_visible/position are nullable (null = "inherit the global
+    // branding toggle"), but the validator used .optional() without
+    // { nullable: true }, so an explicit null was rejected with 400.
+    it('accepts hero_logo_visible: null and stores NULL (inherit)', async () => {
+      const id = await insertEvent(db, adminId, { hero_logo_visible: 1 });
+      const res = await auth(request(app).put(`/api/admin/events/${id}`)).send({
+        hero_logo_visible: null,
+      });
+      expect(res.status).toBe(200);
+      const row = await db('events').where({ id }).first();
+      expect(row.hero_logo_visible).toBeNull();
+    });
+
+    it('still rejects a non-boolean hero_logo_visible', async () => {
+      const id = await insertEvent(db, adminId);
+      const res = await auth(request(app).put(`/api/admin/events/${id}`)).send({
+        hero_logo_visible: 'maybe',
+      });
+      expect(res.status).toBe(400);
+    });
   });
 
   describe('DELETE /:id', () => {