feat(api): admin system metrics endpoint with disk/mem/cpu alerts (0.40.0)

Adds GET /api/v1/admin/system returning disk (scoped to storage_dir
via statvfs), memory, CPU, and a server-side alerts array that fires
at >90% disk or memory.

Disk uses nix::sys::statvfs directly rather than sysinfo's Disks API
to avoid mountpoint-matching gymnastics for the storage_dir. A new
`Storage::local_root() -> Option<&Path>` trait method exposes the
root; the default returns None so a future S3Storage gets `disk:
null` in the response instead of fabricated numbers.

CPU is sampled inline (refresh → 250ms sleep → refresh → read) so the
endpoint adds 250ms of latency per call. No background-cache yet —
admin traffic is low-volume and the moving parts aren't worth it
until polling shows up.

Alerts are evaluated server-side so the frontend can render them
without re-implementing the thresholds.

backend/src/api/admin/mod.rs

@@ -5,6 +5,7 @@
 //! `crate::auth::extractor::RequireAdmin`).
 
 pub mod mangas;
+pub mod system;
 pub mod users;
 
 use axum::Router;
@@ -12,5 +13,8 @@ use axum::Router;
 use crate::app::AppState;
 
 pub fn routes() -> Router<AppState> {
-    Router::new().merge(users::routes()).merge(mangas::routes())
+    Router::new()
+        .merge(users::routes())
+        .merge(mangas::routes())
+        .merge(system::routes())
 }