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>

lib/tokens.ts

@@ -8,6 +8,7 @@
 
 import crypto from 'crypto';
 import { getConfig } from './config';
+import { isUuid } from './uuid';
 
 let _key: Buffer | null = null;
 
@@ -72,8 +73,7 @@ export function decodeAssetId(token: string): string | null {
     }
 
     // Validate it looks like a UUID
-    const uuidRegex = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
-    return uuidRegex.test(decrypted) ? decrypted : null;
+    return isUuid(decrypted) ? decrypted : null;
   } catch {
     return null;
   }