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/[...path]/page.tsx

@@ -25,6 +25,7 @@ import {
   assetExifSummary,
   assetAspectRatio,
 } from '@/lib/urls';
+import { encodeAssetId } from '@/lib/tokens';
 import { getConfig, type GridConfig } from '@/lib/config';
 import { isProtected, isAuthenticated } from '@/lib/auth';
 import PasswordGate from '@/components/PasswordGate';
@@ -96,7 +97,9 @@ function toPhotoItems(assets: ImmichAsset[], showExif: boolean): PhotoItem[] {
       const exif = showExif && a.type === 'IMAGE' ? assetExifSummary(a) : undefined;
       const isVideo = a.type === 'VIDEO';
       return {
-        id: a.id,
+        // Opaque token, never the raw Immich UUID — this object is serialized
+        // into the RSC payload and shipped to the browser. Used only as a React key.
+        id: encodeAssetId(a.id),
         type: isVideo ? 'video' : 'image',
         thumbUrl: imageUrl(a.id, 'preview'),
         previewUrl: imageUrl(a.id, 'preview'),