feat(maintenance): add Docker build-cache prune action

Add an admin-only POST /api/docker/prune-build-cache endpoint plus a Settings > Maintenance danger-zone button to reclaim disk used by the Docker build cache (image + static-site builds), which previously grew unbounded with no UI lever. Prunes unused-only (all=false) so a warm cache is preserved for apps redeploying soon. Mirrors the existing prune-images vertical slice; full en/ru i18n parity.

web/src/lib/i18n/en.json

@@ -123,6 +123,12 @@
     "pruneResult": "Removed {count} images, reclaimed {mb} MB",
     "pruneConfirmMessage": "This will remove unused Docker images belonging to your projects. Images used by active instances will not be affected.",
     "pruneFailed": "Failed to prune images",
+    "buildCacheCleanupHelp": "Reclaim disk used by Docker build cache from image and static-site builds. Only unused cache is removed — apps redeploying still hit any cache that is still in use.",
+    "pruneBuildCache": "Prune Build Cache",
+    "pruningCache": "Pruning...",
+    "pruneCacheResult": "Removed {count} build-cache records, reclaimed {mb} MB",
+    "pruneBuildCacheConfirmMessage": "This removes unused Docker build-cache layers daemon-wide. Layers still in use are kept, so apps that redeploy soon keep their warm cache; others rebuild from scratch on their next deploy.",
+    "pruneCacheFailed": "Failed to prune build cache",
     "proxyProvider": "Proxy Provider",
     "proxyProviderHelp": "Select how reverse proxy routes are managed for deployed containers.",
     "proxyNone": "None",