feat(storage): add S3 backend support and advanced upload limits

- Introduce S3-compatible storage backend support using minio-go.
- Add configuration options for local storage limits, box limits, and rate limiting.
- Implement storage backend selection (local vs S3) for anonymous and registered users.
- Add an `/admin/storage` management interface.
- Update documentation and environment examples with the new configuration variables.

README.md

@@ -22,6 +22,17 @@ Upload policy defaults are also configured in megabytes and can later be changed
 - `WARPBOX_USER_DAILY_UPLOAD_MB=8192`
 - `WARPBOX_DEFAULT_USER_STORAGE_MB=51200`
 - `WARPBOX_USAGE_RETENTION_DAYS=30`
+- `WARPBOX_LOCAL_STORAGE_MAX_GB=100`
+- `WARPBOX_ANONYMOUS_MAX_DAYS=30`
+- `WARPBOX_USER_MAX_DAYS=90`
+- `WARPBOX_ANONYMOUS_DAILY_BOXES=100`
+- `WARPBOX_USER_DAILY_BOXES=250`
+- `WARPBOX_ANONYMOUS_ACTIVE_BOXES=500`
+- `WARPBOX_USER_ACTIVE_BOXES=1000`
+- `WARPBOX_SHORT_WINDOW_REQUESTS=60`
+- `WARPBOX_SHORT_WINDOW_SECONDS=60`
+- `WARPBOX_ANONYMOUS_STORAGE_BACKEND=local`
+- `WARPBOX_USER_STORAGE_BACKEND=local`
 
 Runtime data is configured with `WARPBOX_DATA_DIR` and defaults to `./data` in the dev environment.
 The dev script resolves that path from the repository root.
@@ -126,6 +137,11 @@ from `examples/sharex/warpbox-anonymous.sxcu`; update `RequestURL` to match your
 - `/admin/settings` controls anonymous uploads, anonymous max upload size, daily upload caps, default
   user storage quota, and usage retention.
 - `/admin/users` shows storage/daily usage and lets admins set per-user storage quota overrides.
+- `/admin/storage` manages the built-in local file backend and S3-compatible bucket backends.
+- Upload limits now include daily bytes, daily box counts, active box counts, short-window request
+  limits, max expiration days, local storage capacity in GB, and per-user policy overrides.
+- Uploaded file content, thumbnails, and private box metadata use the selected storage backend.
+  The bbolt database and JSON logs remain local under `./data/db` and `./data/logs`.
 - Anonymous uploads, ShareX uploads, unlisted public box links, password protection, expiry, delete
   tokens, thumbnails, and cleanup continue to work as before.
 
@@ -136,8 +152,8 @@ support will power public forgot-password and optional email delivery.
 
 Warpbox keeps local runtime data under the configured data directory:
 
-- `data/files/{box_id}/@each@{file_id}.ext` - uploaded file contents.
-- `data/files/{box_id}/@thumb@{file_id}.jpg` - generated previews where available.
+- `data/files/{box_id}/@each@{file_id}.ext` - uploaded file contents when the local backend is selected.
+- `data/files/{box_id}/@thumb@{file_id}.jpg` - generated previews when the local backend is selected.
 - `data/db/warpbox.bbolt` - bbolt metadata database for boxes and file records.
 - `data/db/warpbox.bbolt` also stores users, sessions, invites, and collections.
 - `data/db/warpbox.bbolt` stores upload policy settings and daily usage records keyed by plain IP