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/map/route.ts

@@ -14,10 +14,12 @@ import { checkRateLimit, getClientIp } from '@/lib/rate-limit';
 
 export const dynamic = 'force-dynamic';
 
+/** Map data is cached client-side, so a low limit is plenty. */
+const MAP_RPM = 120;
+
 export async function GET(request: NextRequest) {
-  // Rate limit: 120 requests/minute per IP (map data is cached client-side; this guards against abuse)
   const ip = getClientIp(request);
-  const rl = checkRateLimit(ip, 120);
+  const rl = checkRateLimit(`map:${ip}`, MAP_RPM);
   if (!rl.success) {
     return NextResponse.json(
       { error: 'Too many requests' },
@@ -43,28 +45,50 @@ export async function GET(request: NextRequest) {
 
   const locations = await getMapData();
 
-  // Filter locations and albums based on auth
-  const authCache = new Map<string, boolean>();
+  // Filter locations and albums based on auth.
+  // An album is visible only if BOTH its subpage gate (if any) and its own
+  // album-level password gate are satisfied. Standalone albums are NOT
+  // implicitly public — they can carry their own password.
+  const subpageAuthCache = new Map<string, boolean>();
+  const albumAuthCache = new Map<string, boolean>();
+
+  const isSubpageAllowed = (slug: string): boolean => {
+    const cached = subpageAuthCache.get(slug);
+    if (cached !== undefined) return cached;
+    const result = isAuthenticated(slug, getCookie);
+    subpageAuthCache.set(slug, result);
+    return result;
+  };
+
+  const isAlbumAllowed = (albumId: string): boolean => {
+    const cached = albumAuthCache.get(albumId);
+    if (cached !== undefined) return cached;
+    const result = isAuthenticated(albumId, getCookie, 'album');
+    albumAuthCache.set(albumId, result);
+    return result;
+  };
+
   const publicLocations = locations
     .map((loc) => {
-      // Only keep albums the user is allowed to see
-      const allowedAlbums = loc.albums.filter((a) => {
-        if (!a.subpageSlug) return true; // Standalone albums are public
-        if (authCache.has(a.subpageSlug)) return authCache.get(a.subpageSlug)!;
-        const result = isAuthenticated(a.subpageSlug, getCookie);
-        authCache.set(a.subpageSlug, result);
-        return result;
-      });
+      const allowedAlbums = loc.albums.filter(
+        (a) => (!a.subpageSlug || isSubpageAllowed(a.subpageSlug)) && isAlbumAllowed(a.id),
+      );
 
       if (allowedAlbums.length === 0) return null;
 
+      // Aggregate over visible albums only, so neither the marker position,
+      // the photo count, nor the cover asset reveals a protected album.
+      const photoCount = allowedAlbums.reduce((sum, a) => sum + a.photoCount, 0);
+      const latSum = allowedAlbums.reduce((sum, a) => sum + a.latSum, 0);
+      const lngSum = allowedAlbums.reduce((sum, a) => sum + a.lngSum, 0);
+
       return {
         city: loc.city,
         country: loc.country,
-        lat: loc.lat,
-        lng: loc.lng,
-        photoCount: loc.photoCount, // Note: This count might be slightly off if some photos are in hidden albums
-        coverUrl: imageUrl(loc.coverAssetId, 'thumbnail'),
+        lat: latSum / photoCount,
+        lng: lngSum / photoCount,
+        photoCount,
+        coverUrl: imageUrl(allowedAlbums[0].coverAssetId, 'thumbnail'),
         albums: allowedAlbums.map((a) => ({
           name: a.name,
           url: a.subpageSlug ? `/${a.subpageSlug}/${a.slug}` : `/${a.slug}`,