hub v0.23.0: host root-disk pressure monitoring + alert

New HostDiskChecker on the 60s sweep alerts the operator when a Proxmox host root
filesystem crosses warn (90%) / crit (95%). Born/persistent (a disk already full at
hub restart alerts on cycle 1); distinct host_disk_* event types from the guest disk_*;
critical band maps to severity error (the dispatcher only routes warning/error).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HxLA1mZurFq9kt8hneFeCs

hub/internal/store/store.go

@@ -1683,6 +1683,54 @@ type HostLeafRow struct {
 	LeafFP     string
 }
 
+// HostDiskRow is the per-host root-filesystem usage the HostDiskChecker reads: the denormalized
+// disk_percent column (the threshold signal) plus total/used bytes parsed from report_json (event detail
+// only). DiskPercent is the HOST root fs (agent HostMetrics) — distinct from the controller's GUEST cgroup
+// disk. A NULL/absent disk_percent (a pre-disk-reporting agent) yields 0 → the checker treats it as ok.
+type HostDiskRow struct {
+	HostID         string
+	CustomerID     string
+	DiskPercent    float64
+	DiskTotalBytes int64
+	DiskUsedBytes  int64
+}
+
+// GetHostDiskUsage returns the latest root-fs usage per host (MAX(id) per host, mirroring
+// GetHostCapabilities / GetHostLeafFingerprints). disk_percent comes from the denorm column; total/used
+// bytes are parsed from the report body's host block for the event detail (no schema migration needed).
+func (s *Store) GetHostDiskUsage() ([]HostDiskRow, error) {
+	rows, err := s.db.Query(`
+		SELECT hr.host_id, hr.customer_id, hr.disk_percent, hr.report_json
+		FROM host_reports hr
+		JOIN (SELECT host_id, MAX(id) AS mx FROM host_reports GROUP BY host_id) latest
+		  ON hr.id = latest.mx`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var out []HostDiskRow
+	for rows.Next() {
+		var r HostDiskRow
+		var dp sql.NullFloat64
+		var reportJSON string
+		if err := rows.Scan(&r.HostID, &r.CustomerID, &dp, &reportJSON); err != nil {
+			return nil, err
+		}
+		r.DiskPercent = dp.Float64
+		var body struct {
+			Host struct {
+				DiskTotalBytes int64 `json:"disk_total_bytes"`
+				DiskUsedBytes  int64 `json:"disk_used_bytes"`
+			} `json:"host"`
+		}
+		_ = json.Unmarshal([]byte(reportJSON), &body) // malformed/old body → zero bytes (detail only)
+		r.DiskTotalBytes = body.Host.DiskTotalBytes
+		r.DiskUsedBytes = body.Host.DiskUsedBytes
+		out = append(out, r)
+	}
+	return out, rows.Err()
+}
+
 // GetHostLeafFingerprints returns the latest reported local-API leaf fp per host (mirrors
 // GetHostCapabilities — MAX(id) per host, parsed from report_json so there is no schema migration).
 func (s *Store) GetHostLeafFingerprints() ([]HostLeafRow, error) {