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.

backend/config/urls.py

@@ -4,6 +4,7 @@ from django.contrib import admin
 from django.urls import include, path
 
 from apps.core.views import HealthView
+from apps.library.views import RandomItemView
 
 urlpatterns = [
     path("admin/", admin.site.urls),
@@ -15,6 +16,10 @@ urlpatterns = [
     # Liveness probe for uptime monitors (no /api prefix, no auth).
     path("health", HealthView.as_view(), name="health"),
     path("health/", HealthView.as_view()),
+    # Short alias for shell greeting scripts (fish_greeting + fastfetch);
+    # /api/random/ is the canonical path.
+    path("random", RandomItemView.as_view(), name="random"),
+    path("random/", RandomItemView.as_view()),
 ]
 
 if settings.DEBUG: