v0.59.0: report backing device + capacity for a registry-sourced /disks row

Agent-view showed "—" device + no size for a raw (no-PVE-storage) drive because
the registry union row never set backing_device/total_bytes/used_bytes (Observe
drives get those from pvesm status). Resolve BackingDevice via ByUUIDDevicePath +
read capacity via statfsCapacity (build-tagged syscall.Statfs; no-op off-Linux).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/localapi/capacity_linux.go

@@ -0,0 +1,26 @@
+//go:build linux
+
+package localapi
+
+import "syscall"
+
+// statfsCapacity returns the total + used bytes of the filesystem mounted at path (Linux statvfs). A
+// registry-sourced drive (no PVE storage) has no `pvesm status` usage, so /disks reads capacity here
+// instead. ok=false on any error → the caller leaves the size fields zero.
+func statfsCapacity(path string) (total, used int64, ok bool) {
+	var st syscall.Statfs_t
+	if err := syscall.Statfs(path, &st); err != nil {
+		return 0, 0, false
+	}
+	bs := int64(st.Bsize)
+	total = int64(st.Blocks) * bs
+	if total <= 0 {
+		return 0, 0, false
+	}
+	free := int64(st.Bfree) * bs
+	used = total - free
+	if used < 0 {
+		used = 0
+	}
+	return total, used, true
+}