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/jobs.go

@@ -0,0 +1,75 @@
+package jobs
+
+import (
+	"log/slog"
+	"sync"
+	"time"
+
+	"warpbox.dev/backend/libs/config"
+	"warpbox.dev/backend/libs/services"
+)
+
+type job struct {
+	name     string
+	enabled  bool
+	interval time.Duration
+	run      func()
+}
+
+func StartAll(cfg config.Config, logger *slog.Logger, uploadService *services.UploadService) func() {
+	if !cfg.JobsEnabled {
+		logger.Info("background jobs disabled", "source", "jobs", "severity", "dev")
+		return func() {}
+	}
+
+	stops := []func(){
+		start(newCleanupJob(cfg, logger, uploadService), logger),
+		start(newThumbnailsJob(cfg, logger, uploadService), logger),
+	}
+
+	var once sync.Once
+	return func() {
+		once.Do(func() {
+			for _, stop := range stops {
+				stop()
+			}
+		})
+	}
+}
+
+func start(j job, logger *slog.Logger) func() {
+	if !j.enabled {
+		logger.Info("background job disabled", "source", "jobs", "severity", "dev", "job", j.name)
+		return func() {}
+	}
+	if j.interval <= 0 {
+		logger.Info("background job disabled by interval", "source", "jobs", "severity", "dev", "job", j.name, "interval", j.interval.String())
+		return func() {}
+	}
+
+	stop := make(chan struct{})
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		j.run()
+
+		ticker := time.NewTicker(j.interval)
+		defer ticker.Stop()
+		for {
+			select {
+			case <-ticker.C:
+				j.run()
+			case <-stop:
+				return
+			}
+		}
+	}()
+
+	var once sync.Once
+	return func() {
+		once.Do(func() {
+			close(stop)
+			<-done
+		})
+	}
+}