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

web/src/lib/api.ts

@@ -289,6 +289,12 @@ export function listProjectImages(projectId: string): Promise<LocalImage[]> {
 	return get<LocalImage[]>(`/api/projects/${projectId}/images`);
 }
 
+export function getUnusedImageStats(): Promise<{
+	total_size_mb: number; count: number; threshold_mb: number; exceeded: boolean;
+}> {
+	return get<{ total_size_mb: number; count: number; threshold_mb: number; exceeded: boolean }>('/api/docker/unused-images');
+}
+
 export function pruneImages(): Promise<{ images_removed: number; space_reclaimed_mb: number }> {
 	return post<{ images_removed: number; space_reclaimed_mb: number }>('/api/docker/prune-images');
 }