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/api/settings.go

@@ -37,6 +37,7 @@ type settingsRequest struct {
 	CloudflareAPIToken   string  `json:"cloudflare_api_token"`
 	CloudflareZoneID     *string `json:"cloudflare_zone_id,omitempty"`
 	NpmAccessListID      *int    `json:"npm_access_list_id,omitempty"`
+	ImagePruneThresholdMB *int   `json:"image_prune_threshold_mb,omitempty"`
 	NpmRemote            *bool   `json:"npm_remote,omitempty"`
 	ProxyProvider        *string `json:"proxy_provider,omitempty"`
 	TraefikEntrypoint    *string `json:"traefik_entrypoint,omitempty"`
@@ -68,6 +69,7 @@ func (s *Server) getSettings(w http.ResponseWriter, r *http.Request) {
 		"npm_email":                 settings.NpmEmail,
 		"has_npm_password":          settings.NpmPassword != "",
 		"npm_remote":               settings.NpmRemote,
+		"image_prune_threshold_mb": settings.ImagePruneThresholdMB,
 		"npm_access_list_id":       settings.NpmAccessListID,
 		"polling_interval":          settings.PollingInterval,
 		"ssl_certificate_id":        settings.SSLCertificateID,
@@ -195,6 +197,9 @@ func (s *Server) updateSettings(w http.ResponseWriter, r *http.Request) {
 		}
 		updated.ProxyProvider = prov
 	}
+	if req.ImagePruneThresholdMB != nil {
+		updated.ImagePruneThresholdMB = *req.ImagePruneThresholdMB
+	}
 	if req.NpmRemote != nil {
 		updated.NpmRemote = *req.NpmRemote
 	}