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/storage/local.rs

@@ -86,6 +86,10 @@ impl Storage for LocalStorage {
         let path: &Path = &self.resolve(key)?;
         Ok(fs::try_exists(path).await?)
     }
+
+    fn local_root(&self) -> Option<&Path> {
+        Some(&self.root)
+    }
 }
 
 #[cfg(test)]