Phase 12: file browser (0.12.0)

Add a full host filesystem browser reachable from the sidebar (/files):
breadcrumb navigation, browse-root chips, show-hidden toggle, and a table
with size/permissions/mtime. Text files open in a Monaco editor (language by
extension); binary/oversized files fall back to download. Admins can create
folders/files, rename, delete (recursive for dirs), upload, and save edits;
download is available to all users. Every mutation is audit-logged.

Backend: new services/file_service.py reuses device_service's sandbox helpers
(confined to ALLOWED_BROWSE_ROOTS, mapped via HOST_ROOT_PREFIX) and rejects
path traversal and deleting a browse root. routers/files.py exposes
/api/files/{list,read,download,write,mkdir,touch,rename,upload,DELETE}
(reads: any user; mutations: admin). device_service.browse entries gained
mtime + symlink (non-breaking).

Deployment: ALLOWED_BROWSE_ROOTS + HOST_ROOT_PREFIX are now env-wired in
docker-compose.yml and .env.example, with a commented /:/host_root mount to
browse/manage the real host filesystem.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -17,3 +17,12 @@ NOTIFY_WEBHOOKS=
 
 # Throwaway image used to read/write named-volume contents during backups.
 BACKUP_HELPER_IMAGE=alpine:latest
+
+# File browser (sidebar) + volume host-path picker.
+# ALLOWED_BROWSE_ROOTS: comma-separated paths the browser may reach (sandbox).
+# HOST_ROOT_PREFIX: where the host filesystem is mounted inside the backend
+# container. Leave empty to browse the container's own filesystem. To browse
+# the real host, uncomment the "/:/host_root" volume in docker-compose.yml and
+# set HOST_ROOT_PREFIX=/host_root here (mount without :ro to allow edits).
+ALLOWED_BROWSE_ROOTS=/,/mnt,/media,/srv,/opt
+HOST_ROOT_PREFIX=