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/config/config.go

@@ -11,32 +11,38 @@ import (
 )
 
 type Config struct {
-	AppName       string
-	Environment   string
-	Addr          string
-	BaseURL       string
-	DataDir       string
-	StaticDir     string
-	TemplateDir   string
-	ReadTimeout   time.Duration
-	WriteTimeout  time.Duration
-	IdleTimeout   time.Duration
-	MaxUploadSize int64
+	AppName        string
+	Environment    string
+	Addr           string
+	BaseURL        string
+	DataDir        string
+	AdminToken     string
+	StaticDir      string
+	TemplateDir    string
+	ReadTimeout    time.Duration
+	WriteTimeout   time.Duration
+	IdleTimeout    time.Duration
+	CleanupEvery   time.Duration
+	ThumbnailEvery time.Duration
+	MaxUploadSize  int64
 }
 
 func Load() (Config, error) {
 	cfg := Config{
-		AppName:       envString("WARPBOX_APP_NAME", "warpbox.dev"),
-		Environment:   envString("WARPBOX_ENV", "development"),
-		Addr:          envString("WARPBOX_ADDR", ":8080"),
-		BaseURL:       strings.TrimRight(envString("WARPBOX_BASE_URL", "http://localhost:8080"), "/"),
-		DataDir:       envString("WARPBOX_DATA_DIR", defaultPath("data")),
-		StaticDir:     envString("WARPBOX_STATIC_DIR", defaultPath("static")),
-		TemplateDir:   envString("WARPBOX_TEMPLATE_DIR", defaultPath("templates")),
-		ReadTimeout:   envDuration("WARPBOX_READ_TIMEOUT", 15*time.Second),
-		WriteTimeout:  envDuration("WARPBOX_WRITE_TIMEOUT", 60*time.Second),
-		IdleTimeout:   envDuration("WARPBOX_IDLE_TIMEOUT", 120*time.Second),
-		MaxUploadSize: envMegabytes("WARPBOX_MAX_UPLOAD_SIZE_MB", 2048), // 2 GiB default.
+		AppName:        envString("WARPBOX_APP_NAME", "warpbox.dev"),
+		Environment:    envString("WARPBOX_ENV", "development"),
+		Addr:           envString("WARPBOX_ADDR", ":8080"),
+		BaseURL:        strings.TrimRight(envString("WARPBOX_BASE_URL", "http://localhost:8080"), "/"),
+		DataDir:        envString("WARPBOX_DATA_DIR", defaultPath("data")),
+		AdminToken:     envString("WARPBOX_ADMIN_TOKEN", ""),
+		StaticDir:      envString("WARPBOX_STATIC_DIR", defaultPath("static")),
+		TemplateDir:    envString("WARPBOX_TEMPLATE_DIR", defaultPath("templates")),
+		ReadTimeout:    envDuration("WARPBOX_READ_TIMEOUT", 15*time.Second),
+		WriteTimeout:   envDuration("WARPBOX_WRITE_TIMEOUT", 60*time.Second),
+		IdleTimeout:    envDuration("WARPBOX_IDLE_TIMEOUT", 120*time.Second),
+		CleanupEvery:   envDuration("WARPBOX_CLEANUP_EVERY", time.Hour),
+		ThumbnailEvery: envDuration("WARPBOX_THUMBNAIL_EVERY", time.Minute),
+		MaxUploadSize:  envMegabytes("WARPBOX_MAX_UPLOAD_SIZE_MB", 2048), // 2 GiB default.
 	}
 
 	if cfg.BaseURL == "" {