security: fix pre-auth bypasses and restore Immich 3.x compatibility (#378)

Seven security issues from an audit, plus album pages and the map coming
up empty against Immich 3.x. Verified against a real Immich 3.0.3
instance in a production build.

Security:
- Admin session forgery via a hardcoded 'dev-fallback-secret' signing key
  that bypassed the production AUTH_SECRET guard
- /api/map ignored album passwords and returned working asset tokens;
  rewriting ?size=thumbnail to ?size=original downloaded full-resolution
  originals from protected albums
- Admin login had no rate limit, and the client IP came from a spoofable
  header, so rotating it bypassed every limit in the app
- Raw Immich asset UUIDs were serialized into the RSC payload
- Album tokens never expired server-side; a captured token replayed forever
- Path traversal into the Immich API via an unvalidated album id
- /admin ran with no enforced CSP (only Report-Only reached it)

TRUSTED_PROXIES is replaced by TRUSTED_PROXY_HOPS. The old variable
matched proxy IPs against the socket peer address, which self-hosted
Next.js never exposes, so the check could never pass: unset, the limiter
read a spoofable header; set, every request shared one bucket and a single
client could rate-limit the whole site. Migrates automatically.

Immich 3.x: GET /albums/:id no longer embeds assets. Assets now come from
POST /search/metadata with withExif: true — without that flag EXIF is
omitted and grid aspect ratios, the lightbox panel and map markers stay
empty. Drops support for Immich below 3.0, which now logs a warning.

Upgrade: requires Immich 3.0+; set TRUSTED_PROXY_HOPS (nginx = 1);
visitors of protected areas and admin sessions must log in once more.

91 unit tests (25 new); the new admin-auth and rate-limit cases were
confirmed to fail against the pre-fix code.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/api/admin/auth/route.ts

@@ -7,9 +7,25 @@ import {
   COOKIE_NAME,
   SESSION_DURATION_MS,
 } from '@/lib/admin/auth';
+import { checkRateLimit, getClientIp } from '@/lib/rate-limit';
+
+/** Admin login attempts per minute per IP — the highest-value credential in the app. */
+const ADMIN_AUTH_RPM = 5;
 
 /** POST: Login with admin password. */
 export async function POST(request: NextRequest) {
+  const ip = getClientIp(request);
+  const rl = checkRateLimit(`admin-auth:${ip}`, ADMIN_AUTH_RPM);
+  if (!rl.success) {
+    return NextResponse.json(
+      { error: 'Too many requests' },
+      {
+        status: 429,
+        headers: { 'Retry-After': String(Math.ceil((rl.resetAt - Date.now()) / 1000)) },
+      },
+    );
+  }
+
   if (!isAdminEnabled()) {
     return NextResponse.json(
       { error: 'Admin panel is not enabled. Set ADMIN_PASSWORD in your environment.' },