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.

.env.example

@@ -4,9 +4,12 @@ WARPBOX_ADDR=:8080
 WARPBOX_BASE_URL=http://localhost:8080
 WARPBOX_DATA_DIR=./data
 WARPBOX_ADMIN_TOKEN=change-me
+WARPBOX_JOBS_ENABLED=true
+WARPBOX_CLEANUP_ENABLED=true
 WARPBOX_CLEANUP_EVERY=1h
+WARPBOX_THUMBNAIL_ENABLED=true
 WARPBOX_THUMBNAIL_EVERY=1m
-WARPBOX_MAX_UPLOAD_SIZE_MB=2048
+WARPBOX_MAX_UPLOAD_SIZE_MB=16384
 WARPBOX_READ_TIMEOUT=15s
 WARPBOX_WRITE_TIMEOUT=60s
 WARPBOX_IDLE_TIMEOUT=120s