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/store.go

@@ -121,6 +121,8 @@ func (s *Store) runMigrations() error {
 		`ALTER TABLE projects ADD COLUMN npm_access_list_id INTEGER NOT NULL DEFAULT 0`,
 		// Separate public IP for DNS A records.
 		`ALTER TABLE settings ADD COLUMN public_ip TEXT NOT NULL DEFAULT ''`,
+		// Image prune threshold (MB). Warn on dashboard when exceeded. 0 = disabled.
+		`ALTER TABLE settings ADD COLUMN image_prune_threshold_mb INTEGER NOT NULL DEFAULT 1024`,
 	}
 
 	for _, m := range migrations {
@@ -225,6 +227,7 @@ CREATE TABLE IF NOT EXISTS settings (
 	base_volume_path   TEXT NOT NULL DEFAULT '',
 	ssl_certificate_id     INTEGER NOT NULL DEFAULT 0,
 	npm_remote             INTEGER NOT NULL DEFAULT 0,
+	image_prune_threshold_mb INTEGER NOT NULL DEFAULT 1024,
 	npm_access_list_id     INTEGER NOT NULL DEFAULT 0,
 	traefik_entrypoint     TEXT NOT NULL DEFAULT 'websecure',
 	traefik_cert_resolver  TEXT NOT NULL DEFAULT 'letsencrypt',