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_test.go

@@ -0,0 +1,50 @@
+package jobs
+
+import (
+	"testing"
+	"time"
+
+	"warpbox.dev/backend/libs/services"
+)
+
+func TestShouldDeleteBox(t *testing.T) {
+	now := time.Date(2026, 5, 29, 12, 0, 0, 0, time.UTC)
+
+	tests := map[string]struct {
+		box  services.Box
+		want bool
+	}{
+		"expired": {
+			box:  services.Box{ExpiresAt: now.Add(-time.Second)},
+			want: true,
+		},
+		"expires now": {
+			box:  services.Box{ExpiresAt: now},
+			want: true,
+		},
+		"download limit reached": {
+			box:  services.Box{ExpiresAt: now.Add(time.Hour), MaxDownloads: 3, DownloadCount: 3},
+			want: true,
+		},
+		"download limit exceeded": {
+			box:  services.Box{ExpiresAt: now.Add(time.Hour), MaxDownloads: 3, DownloadCount: 4},
+			want: true,
+		},
+		"active unlimited": {
+			box:  services.Box{ExpiresAt: now.Add(time.Hour)},
+			want: false,
+		},
+		"active under limit": {
+			box:  services.Box{ExpiresAt: now.Add(time.Hour), MaxDownloads: 3, DownloadCount: 2},
+			want: false,
+		},
+	}
+
+	for name, tt := range tests {
+		t.Run(name, func(t *testing.T) {
+			if got := shouldDeleteBox(tt.box, now); got != tt.want {
+				t.Fatalf("shouldDeleteBox() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}