feat(alerts): add per-volume disk thresholds for host agents

Allow users to set custom disk usage thresholds per mounted filesystem
on host agents, rather than applying a single threshold to all volumes.

This addresses NAS/NVR use cases where some volumes (e.g., NVR storage)
intentionally run at 99% while others need strict monitoring.

Backend:
- Check for disk-specific overrides before using HostDefaults.Disk
- Override key format: host:<hostId>/disk:<mountpoint>
- Support both custom thresholds and disable per-disk

Frontend:
- Add 'hostDisk' resource type
- Add "Host Disks" collapsible section in Thresholds → Hosts tab
- Group disks by host for easier navigation

Closes #1103

go.mod

@@ -10,6 +10,7 @@ require (
 	github.com/crewjam/saml v0.5.1
 	github.com/docker/docker v28.5.2+incompatible
 	github.com/fsnotify/fsnotify v1.9.0
+	github.com/go-pdf/fpdf v0.9.0
 	github.com/google/uuid v1.6.0
 	github.com/gorilla/websocket v1.5.3
 	github.com/joho/godotenv v1.5.1
@@ -60,9 +61,7 @@ require (
 	github.com/go-openapi/jsonpointer v0.21.0 // indirect
 	github.com/go-openapi/jsonreference v0.20.2 // indirect
 	github.com/go-openapi/swag v0.23.0 // indirect
-	github.com/go-pdf/fpdf v0.9.0 // indirect
 	github.com/gogo/protobuf v1.3.2 // indirect
-	github.com/golang-jwt/jwt/v4 v4.5.2 // indirect
 	github.com/golang/protobuf v1.5.4 // indirect
 	github.com/google/gnostic-models v0.6.8 // indirect
 	github.com/google/go-cmp v0.7.0 // indirect