Add a Random image endpoint and SPA page (with fastfetch mode)

Backend: GET /api/random/ (aliases /random and /random/) returns a random
library image with:
- rating=s,q,e filtering (comma separated, default any);
- fastfetch mode (?fastfetch=1 or any User-Agent containing "fastfetch")
  that only considers png/jpg/gif - what terminal viewers can show;
- JSON with j_id, filename, extension, rating, size, e621 id plus absolute
  url/download_url/thumbnail_url. Authenticated callers get signed URLs so
  fastfetch and image viewers can load them without headers; guests get
  unsigned URLs and never receive hidden_from_guests items.

Tests: apps/library/tests/test_random.py (8 tests) covering the response
contract, guest signatures, image-only default, the fastfetch format
restriction (flag and User-Agent), rating filters, guest visibility and the
short alias.

Frontend: /random page with rating pills, R to roll, Open/Download and a
library link, plus navigation and command palette entries; needs a backend,
hidden in local mode.

nginx: /random negotiates on Accept so browsers keep getting the SPA while
scripts get the JSON (verified with the proxy and frontend containers).

Also fixes a regression from the SSRF change: the guest download proxy
still referenced the removed 'parsed' variable on its success path, so
every proxied download would have 500'd. Redirect hops are now covered by
tests with a mocked requests.get.

frontend/src/lib/types.ts

@@ -1,3 +1,18 @@
+export interface RandomItem {
+  j_id: string;
+  md5: string;
+  filename: string;
+  extension: string;
+  kind: "image";
+  rating: string;
+  size: number;
+  e621_post_id: number | null;
+  url: string;
+  download_url: string;
+  thumbnail_url: string;
+  fastfetch: boolean;
+}
+
 export interface UserPreferences {
   landing_page?: "library" | "online" | "pools" | "followed";
   ratings?: string[];