feat(gallery): admin preview skips the password on protected galleries (#981)

Closes #868.

A logged-in admin opening a published, password-protected gallery is let
straight in, mirroring the existing draft-visibility bypass.

Mechanism: an explicit ?admin_preview=1 intent flag AND a verified admin session
read from the httpOnly admin_token cookie (or an admin-typed Bearer) — never a
token from the URL. This retires the old ?preview=<raw-admin-JWT> scheme, which
leaked a 24h admin token into the address bar, referrers and proxy logs.

Per-request bypass only: no gallery JWT is minted, the password endpoint is
never reached so the login_attempts lockout buckets stay clean, and admin
previews are excluded from guest analytics (access_logs, download counts,
per-photo view_count, notification bells).

Review (two rounds) closed three blockers and two concerns:

- Transport: verifyGalleryAccess now resolves admin preview before any gallery
  credential, and isAdminPreview reads the admin cookie first and type-checks
  every candidate — so an admin Bearer no longer 403s on the type gate, and a
  coexisting gallery session can no longer shadow the admin cookie.
- Reveal mode (#838) is a second consumer of isAdminPreview; its bypass is
  unchanged, only the transport moves. revealMode.test.js updated off the
  retired scheme and now carries a coexisting gallery Bearer.
- Admin previews no longer inflate per-photo view counts, and the internal photo
  redirects preserve the flag via withPreview() so they still authorise.
- Happy path: GalleryPage renders GalleryView directly for a preview instead of
  attempting the public empty-password auto-login, which 401'd against a
  genuinely protected gallery and stranded the page on the skeleton.

The backend job timed out once at the 10-minute CI limit; a re-run completed in
2m02s, in line with main's ~2m10s baseline, so that was a runner flake rather
than a hang.

backend/__tests__/integration/revealMode.test.js

@@ -152,9 +152,13 @@ describe('Reveal mode (#838)', () => {
       expect(res.body.photos).toHaveLength(2);
     });
 
-    it('/photos serves the admin preview everything', async () => {
+    it('/photos serves the admin preview everything (new transport: ?admin_preview=1 + admin cookie, even with a coexisting gallery session)', async () => {
+      // #868/#981: reveal-mode hiding is bypassed for an admin preview via the
+      // new transport (explicit flag + httpOnly admin_token cookie), NOT the
+      // retired ?preview=<jwt>. The coexisting gallery Bearer must not shadow it.
       const res = await request(app)
-        .get(`/api/gallery/${SLUG}/photos?preview=${encodeURIComponent(adminToken)}`)
+        .get(`/api/gallery/${SLUG}/photos?admin_preview=1`)
+        .set('Cookie', [`admin_token=${adminToken}`])
         .set('Authorization', `Bearer ${galleryToken()}`);
       expect(res.status).toBe(200);
       expect(res.body.hidden_until_reveal).toBe(false);