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.

internal/api/docker.go

@@ -348,3 +348,32 @@ func (s *Server) pruneImages(w http.ResponseWriter, r *http.Request) {
 		"space_reclaimed_mb": reclaimedBytes / (1024 * 1024),
 	})
 }
+
+// pruneBuildCache handles POST /api/docker/prune-build-cache. It removes
+// unused Docker build-cache records daemon-wide (all=false), so an app's next
+// rebuild still hits its warm cache. The build cache is regenerable by
+// definition — pruning only forces slower rebuilds, never data loss — and the
+// dockerfile/static deploy paths never reclaim it on teardown, so it grows
+// monotonically until pruned here.
+func (s *Server) pruneBuildCache(w http.ResponseWriter, r *http.Request) {
+	if s.docker == nil {
+		respondError(w, http.StatusServiceUnavailable, "Docker is not available")
+		return
+	}
+
+	result, err := s.docker.PruneBuildCache(r.Context(), false)
+	if err != nil {
+		slog.Error("prune: build cache", "error", err)
+		respondError(w, http.StatusInternalServerError, "internal server error")
+		return
+	}
+
+	slog.Info("prune: build cache",
+		"caches_deleted", result.CachesDeleted,
+		"space_reclaimed_mb", result.SpaceReclaimed/(1024*1024))
+
+	respondJSON(w, http.StatusOK, map[string]any{
+		"caches_deleted":     result.CachesDeleted,
+		"space_reclaimed_mb": result.SpaceReclaimed / (1024 * 1024),
+	})
+}