feat: catalogue enrichment — tagline, genres, warnings, quality score, batch covers

Backend (handlers_catalogue.go):
- POST /api/admin/text-gen/tagline — 1-sentence marketing hook
- POST /api/admin/text-gen/genres + /apply — LLM genre suggestions, editable + persist
- POST /api/admin/text-gen/content-warnings — mature theme detection
- POST /api/admin/text-gen/quality-score — 1–5 description quality rating
- POST /api/admin/catalogue/batch-covers (SSE) — generate covers for books missing one
- POST /api/admin/catalogue/batch-covers/cancel — cancel via in-memory job registry
- POST /api/admin/catalogue/refresh-metadata/{slug} (SSE) — description + cover refresh

Frontend:
- text-gen: 4 new tabs (Tagline, Genres, Warnings, Quality) with book autocomplete
- image-gen: localStorage style presets (save/apply/delete named prompt templates)
- catalogue-tools: new admin page with batch cover SSE progress + cancel
- admin nav: "Catalogue Tools" link added

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/messages/en.json

@@ -402,6 +402,7 @@
   "admin_nav_changelog": "Changelog",
   "admin_nav_image_gen": "Image Gen",
   "admin_nav_text_gen": "Text Gen",
+  "admin_nav_catalogue_tools": "Catalogue Tools",
   "admin_nav_feedback": "Feedback",
   "admin_nav_errors": "Errors",
   "admin_nav_analytics": "Analytics",