feat: add AI Jobs admin page, cache model lists, improve image-gen 502 UX

- Add /admin/ai-jobs page with live-polling jobs table, status badges, progress bars, and cancel action
- Add listAIJobs() helper in pocketbase.ts; AIJob type
- Add POST /api/admin/ai-jobs/[id]/cancel SvelteKit proxy
- Add ai-jobs link to admin sidebar nav (all 5 locales)
- Cache image-gen and text-gen model lists in Valkey (10 min TTL) via scraper.ts helpers
- Cache changelog Gitea API response (5 min TTL)
- Improve 502/504 error message on image-gen page with CF AI timeout hint
- Show CF AI timeout warning in advanced options when a Cloudflare/FLUX model is selected

ui/messages/fr.json

@@ -403,6 +403,7 @@
   "admin_nav_image_gen": "Image Gen",
   "admin_nav_text_gen": "Text Gen",
   "admin_nav_catalogue_tools": "Catalogue Tools",
+  "admin_nav_ai_jobs": "Tâches IA",
   "admin_nav_errors": "Erreurs",
   "admin_nav_analytics": "Analytique",
   "admin_nav_logs": "Journaux",