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.

.env.example

@@ -3,6 +3,9 @@ WARPBOX_ENV=development
 WARPBOX_ADDR=:8080
 WARPBOX_BASE_URL=http://localhost:8080
 WARPBOX_DATA_DIR=./data
+WARPBOX_ADMIN_TOKEN=change-me
+WARPBOX_CLEANUP_EVERY=1h
+WARPBOX_THUMBNAIL_EVERY=1m
 WARPBOX_MAX_UPLOAD_SIZE_MB=2048
 WARPBOX_READ_TIMEOUT=15s
 WARPBOX_WRITE_TIMEOUT=60s