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/i18n/en.json

@@ -30,7 +30,9 @@
     "noProjects": "No projects yet.",
     "addFirst": "Add your first project",
     "loadFailed": "Failed to load dashboard",
-    "staleContainers": "Stale Containers"
+    "staleContainers": "Stale Containers",
+    "unusedImagesWarning": "Unused Docker images are taking up disk space",
+    "unusedImages": "unused images"
   },
   "projects": {
     "title": "Projects",
@@ -257,6 +259,8 @@
     "staleThresholdHelp": "Containers inactive for longer than this will be flagged as stale.",
     "dockerCleanup": "Docker Image Cleanup",
     "dockerCleanupHelp": "Remove unused Docker images belonging to your projects. Only images not used by active instances are removed.",
+    "pruneThreshold": "Warning Threshold (MB)",
+    "pruneThresholdHelp": "Show dashboard warning when unused project images exceed this size. 0 = disabled.",
     "pruneImages": "Prune Unused Images",
     "pruning": "Pruning...",
     "pruneResult": "Removed {count} images, reclaimed {mb} MB",