feat: add admin console, cleanup, and thumbnail workers

- Implement a token-authenticated admin console at `/admin` with overview metrics and file management.
- Add a background worker to periodically clean up expired boxes based on `WARPBOX_CLEANUP_EVERY`.
- Add a background worker to generate image and video thumbnails based on `WARPBOX_THUMBNAIL_EVERY`.
- Update file storage paths to use `@each@` and `@thumb@` prefixes to separate original files from thumbnails.
- Add severity fields to startup logs and update configuration templates.

backend/libs/handlers/app.go

@@ -27,9 +27,18 @@ func NewApp(cfg config.Config, logger *slog.Logger, renderer *web.Renderer, uplo
 
 func (a *App) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("GET /", a.Home)
+	mux.HandleFunc("GET /admin/login", a.AdminLogin)
+	mux.HandleFunc("POST /admin/login", a.AdminLoginPost)
+	mux.HandleFunc("POST /admin/logout", a.AdminLogout)
+	mux.HandleFunc("GET /admin", a.AdminDashboard)
+	mux.HandleFunc("GET /admin/files", a.AdminFiles)
+	mux.HandleFunc("POST /admin/boxes/{boxID}/delete", a.AdminDeleteBox)
 	mux.HandleFunc("GET /d/{boxID}", a.DownloadPage)
+	mux.HandleFunc("POST /d/{boxID}/unlock", a.UnlockBox)
 	mux.HandleFunc("GET /d/{boxID}/zip", a.DownloadZip)
 	mux.HandleFunc("GET /d/{boxID}/f/{fileID}", a.DownloadFile)
+	mux.HandleFunc("GET /d/{boxID}/f/{fileID}/download", a.DownloadFileContent)
+	mux.HandleFunc("GET /d/{boxID}/thumb/{fileID}", a.Thumbnail)
 	mux.HandleFunc("GET /healthz", a.Health)
 	mux.HandleFunc("GET /api/v1/health", a.Health)
 	mux.HandleFunc("POST /api/v1/upload", a.Upload)