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/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.39.0"
+version = "0.40.0"
 edition = "2021"
 default-run = "mangalord"
 
@@ -45,6 +45,8 @@ futures-core = "0.3"
 futures-util = "0.3"
 bytes = "1"
 chromiumoxide = { version = "0.7", features = ["tokio-runtime", "_fetcher-rusttls-tokio"], default-features = false }
+sysinfo = { version = "0.32", default-features = false, features = ["system"] }
+nix = { version = "0.29", features = ["fs"] }
 scraper = "0.20"
 reqwest = { version = "0.12", default-features = false, features = ["rustls-tls", "socks", "cookies", "stream"] }