feat: configurable unused images threshold with dashboard warning

- Add image_prune_threshold_mb setting (default 1024 MB)
- Add GET /api/docker/unused-images endpoint returning unused image count, size, and threshold status
- Dashboard shows amber warning banner when unused project images exceed threshold
- Banner links to settings page for pruning, shows count and human-readable size (MB/GB)
- Threshold configurable in Docker Image Cleanup section of settings
- DB migration + schema for image_prune_threshold_mb

internal/store/models.go

@@ -74,6 +74,7 @@ type Settings struct {
 	TraefikCertResolver string `json:"traefik_cert_resolver"`
 	TraefikNetwork      string `json:"traefik_network"`
 	TraefikAPIURL       string `json:"traefik_api_url"`
+	ImagePruneThresholdMB int  `json:"image_prune_threshold_mb"`
 	BackupEnabled       bool   `json:"backup_enabled"`
 	BackupIntervalHours  int    `json:"backup_interval_hours"`
 	BackupRetentionCount int    `json:"backup_retention_count"`