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.

ROADMAP.md

@@ -21,6 +21,11 @@ they land.
         blacklisted items)
   - [x] Status filter (matched / not_found / deleted / custom / unknown)
 
+- [x] **Random image** endpoint and page: `GET /api/random/` (aliases
+      `/random`, `/random/`) with `rating=s,q,e` filters; fastfetch mode
+      (`?fastfetch=1` or a Fastfetch User-Agent) only returns png/jpg/gif as
+      JSON with a signed absolute link, for the fish_greeting scripts; the
+      `/random` SPA page rolls with rating pills and the `R` key
 - [x] **Ephemeral similarity check** (`/similar`)
   - [x] Drop a file: exact MD5 match, perceptual matches against the library,
         and e621 IQDB candidates (auto-run for images)