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/albums/[albumId]/assets/route.ts

@@ -5,6 +5,7 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { isAdminAuthenticated, isAdminEnabled } from '@/lib/admin/auth';
 import { getConfig } from '@/lib/config';
+import { isUuid } from '@/lib/uuid';
 
 interface ImmichAlbumDetail {
   assets: Array<{
@@ -35,8 +36,14 @@ export async function GET(
 
   const { albumId } = await params;
 
+  // Without this, `..%2f..%2fusers` resolves inside fetch() to an arbitrary
+  // Immich endpoint, called with the server's API key.
+  if (!isUuid(albumId)) {
+    return NextResponse.json({ error: 'Invalid album ID' }, { status: 400 });
+  }
+
   try {
-    const res = await fetch(`${config.immich.apiUrl}/albums/${albumId}`, {
+    const res = await fetch(`${config.immich.apiUrl}/albums/${encodeURIComponent(albumId)}`, {
       headers: {
         'x-api-key': config.immich.apiKey,
         Accept: 'application/json',