feat(v3): Caddy hardening, Watchtower, scrape fixes, docs diagrams

- Caddy: custom image with caddy-ratelimit plugin, security headers
  (X-Frame-Options, HSTS, CSP-adjacent, etc.), per-IP rate limiting on
  auth/scrape/global zones, static error pages (502/503/504), fix routing
  to remove /api/scrape/* and /api/chapter-text-preview/* direct-to-backend
  (were bypassing SvelteKit auth middleware)
- docker-compose: Caddy build context + error volume, Watchtower service
  (label-enable mode, 5 min poll), watchtower labels on backend/runner/ui
- Scraper: ScrapeChapterList uses retryGet (9 attempts, Retry-After backoff)
  to fix 429-induced chapter list failures; upTo param stops pagination early
  for range scrapes
- UI: Browse→Catalogue rename (routes, API, links), admin scrape page
  Continue/Retry buttons, +error.svelte branded error page, type cleanup
  (removed dead exports, added BookPreviewMeta/BookPreviewResponse to scraper.ts)
- Meilisearch: meta_updated field, sort=update fix, facet distribution
- Docs: reorganise into docs/d2/ and docs/mermaid/ subdirectories, update
  all diagrams to reflect Caddy/Watchtower/routing changes, add
  api-routing.d2 ownership map with auth-level colour coding, regenerate SVGs

v3/docs/api-endpoints.md

@@ -1,8 +1,8 @@
 # API Endpoint Reference
 
-All endpoints served by the Go **backend** binary on `:8080`. In production all
-traffic is routed through Caddy — `/api/*` and `/health` are proxied to the
-backend; everything else goes to the SvelteKit UI.
+> **Routing ownership map**: see [`docs/d2/api-routing.svg`](d2/api-routing.svg) (source: [`docs/d2/api-routing.d2`](d2/api-routing.d2)) for a visual overview of which paths Caddy sends to the backend directly vs. through SvelteKit, with auth levels colour-coded.
+
+All traffic enters through **Caddy :443**. Caddy routes a subset of paths directly to the Go backend (bypassing SvelteKit); everything else goes to SvelteKit, which enforces auth before proxying onward.
 
 ## Health / Version