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>

CLAUDE.md

@@ -92,7 +92,7 @@ Per-subpage and per-album password gating using HMAC tokens in `HttpOnly` cookie
 
 ### Rate limiting (`lib/rate-limit.ts`)
 
-In-memory sliding-window rate limiter. Important: **in-memory only** — does not work across multiple Node.js instances. Uses FIFO eviction (not reject-on-full) to prevent DoS via store flooding. Configurable via `RATE_LIMIT_RPM`. Trusted proxy IPs must be set via `TRUSTED_PROXIES` env var to safely trust `X-Forwarded-For` headers.
+In-memory sliding-window rate limiter. Important: **in-memory only** — does not work across multiple Node.js instances. Uses FIFO eviction (not reject-on-full) to prevent DoS via store flooding. Configurable via `RATE_LIMIT_RPM`. `TRUSTED_PROXY_HOPS` must be set to the number of reverse proxies in front of the app (nginx = 1); the client IP is then read that many entries from the right of `X-Forwarded-For`, which proxies append to. Without it the IP comes from a spoofable header. Bucket keys are namespaced per endpoint (`image:`, `map:`, `auth:`, …) so limits don't collide.
 
 ### Theming system