feat(v3): add Dozzle/Uptime Kuma/Gotify; fix Meilisearch catalogue re-index churn

- docker-compose: add dozzle, uptime-kuma, gotify services with Caddy subdomains
  (logs/uptime/push.libnovel.cc); bind-mount dozzle/users.yml for simple auth
- Caddyfile: add site blocks for logs, uptime, push subdomains
- admin layout: add Logs, Uptime, Push tabs to external tools pill
- CatalogueEntry: add Slug field; populated by novelfire scraper
- meili.Client: add BookExists() to check if a book is already indexed
- catalogue_refresh: skip books already present in Meilisearch to prevent
  redundant re-scraping and re-indexing on every runner restart
- docker-compose runner: set RUNNER_SKIP_INITIAL_CATALOGUE_REFRESH=true

v3/backend/internal/domain/domain.go

@@ -28,6 +28,7 @@ type BookMeta struct {
 
 // CatalogueEntry is a lightweight book reference returned by catalogue pages.
 type CatalogueEntry struct {
+	Slug  string `json:"slug"`
 	Title string `json:"title"`
 	URL   string `json:"url"`
 }