feat: implement configurable background jobs and toggle flags

Introduce environment variables to globally and individually control background jobs:
- `WARPBOX_JOBS_ENABLED` to toggle all background workers.
- `WARPBOX_CLEANUP_ENABLED` to toggle the expired box cleanup job.
- `WARPBOX_THUMBNAIL_ENABLED` to toggle the thumbnail generation job.

Refactor background tasks into a dedicated `backend/libs/jobs` package, allowing jobs to be registered, scheduled, and conditionally run based on the new configuration flags. Additionally, update the default maximum upload size in `.env.example` to 16GB and document the new settings in the README.

backend/libs/jobs/cleanup.go

@@ -0,0 +1,57 @@
+package jobs
+
+import (
+	"log/slog"
+	"time"
+
+	"warpbox.dev/backend/libs/config"
+	"warpbox.dev/backend/libs/services"
+)
+
+func newCleanupJob(cfg config.Config, logger *slog.Logger, uploadService *services.UploadService) job {
+	return job{
+		name:     "cleanup",
+		enabled:  cfg.CleanupEnabled,
+		interval: cfg.CleanupEvery,
+		run: func() {
+			cleaned, err := cleanupUnavailableBoxes(uploadService, logger)
+			if err != nil {
+				logger.Warn("cleanup job failed", "source", "housekeeping", "severity", "warn", "code", 4202, "error", err.Error())
+				return
+			}
+			if cleaned > 0 {
+				logger.Info("cleanup job complete", "source", "housekeeping", "severity", "user_activity", "code", 2202, "cleaned", cleaned)
+			}
+		},
+	}
+}
+
+func cleanupUnavailableBoxes(uploadService *services.UploadService, logger *slog.Logger) (int, error) {
+	boxes, err := uploadService.ListBoxes(0)
+	if err != nil {
+		return 0, err
+	}
+
+	now := time.Now().UTC()
+	cleaned := 0
+	for _, box := range boxes {
+		if !shouldDeleteBox(box, now) {
+			continue
+		}
+		if err := uploadService.DeleteBoxWithSource(box.ID, "housekeeping"); err != nil {
+			return cleaned, err
+		}
+		cleaned++
+	}
+	if cleaned > 0 {
+		logger.Info("unavailable boxes cleaned", "source", "housekeeping", "severity", "user_activity", "code", 2201, "cleaned", cleaned)
+	}
+	return cleaned, nil
+}
+
+func shouldDeleteBox(box services.Box, now time.Time) bool {
+	if !box.ExpiresAt.After(now) {
+		return true
+	}
+	return box.MaxDownloads > 0 && box.DownloadCount >= box.MaxDownloads
+}