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/components/CommandPalette.tsx

@@ -2,6 +2,7 @@ import { useQuery } from "@tanstack/react-query";
 import {
   Cable,
   Copy,
+  Dices,
   ExternalLink,
   FolderOpen,
   Globe,
@@ -124,6 +125,12 @@ function CommandPaletteDialog({ onClose }: { onClose: () => void }) {
               icon: FolderOpen,
               run: () => navigate("/"),
             },
+            {
+              id: "random",
+              label: "Roll a random image",
+              icon: Dices,
+              run: () => navigate("/random"),
+            },
           ]
         : []),
       {