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/web/renderer.go

@@ -18,22 +18,23 @@ type PageData struct {
 	BaseURL     string
 	Title       string
 	Description string
+	ImageURL    string
 	CurrentYear int
 	Data        any
 }
 
 func NewRenderer(templateDir, appName, baseURL string) (*Renderer, error) {
-	layouts, err := filepath.Glob(filepath.Join(templateDir, "layouts", "*.gohtml"))
+	layouts, err := filepath.Glob(filepath.Join(templateDir, "layouts", "*.html"))
 	if err != nil {
 		return nil, err
 	}
 
-	partials, err := filepath.Glob(filepath.Join(templateDir, "partials", "*.gohtml"))
+	partials, err := filepath.Glob(filepath.Join(templateDir, "partials", "*.html"))
 	if err != nil {
 		return nil, err
 	}
 
-	pages, err := filepath.Glob(filepath.Join(templateDir, "pages", "*.gohtml"))
+	pages, err := filepath.Glob(filepath.Join(templateDir, "pages", "*.html"))
 	if err != nil {
 		return nil, err
 	}