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/og/route.tsx

@@ -11,11 +11,19 @@ import { NextRequest, NextResponse } from 'next/server';
 import { getConfig } from '@/lib/config';
 import { checkRateLimit, getClientIp } from '@/lib/rate-limit';
 
+/**
+ * OG image rendering runs satori + resvg per request and each unique ?title
+ * misses every cache tier, so this is far more expensive than an image proxy
+ * hit. It must not inherit RATE_LIMIT_RPM (default 1500) — that turns the
+ * endpoint into an unauthenticated CPU amplifier.
+ */
+const OG_RPM = 30;
+
 export async function GET(request: NextRequest) {
   const ip = getClientIp(request);
-  const { theme, rateLimitRpm } = getConfig();
+  const { theme } = getConfig();
 
-  const { success } = checkRateLimit(`og:${ip}`, rateLimitRpm);
+  const { success } = checkRateLimit(`og:${ip}`, OG_RPM);
   if (!success) {
     return new NextResponse('Too many requests', { status: 429 });
   }