Docker build cache: .dockerignore, cache mount, layer reorder, restart script

- .dockerignore: exclude node_modules/.git/tmp/data/logs
- Dockerfile: BuildKit cache mount for npm, --omit=dev at install, drop
  npm prune step
- Reorder layers: shared/src/frontend copy before build, server copy +
  better-sqlite3 rebuild after. Server/shared changes no longer invalidate
  frontend build layer unless shared (FE dep) changes
- Remove duplicate shared COPY in runtime stage (bundled in FE build)
- docker/restart.sh: stop + rebuild + start in one script

server/index.js

@@ -1,6 +1,7 @@
 // server/index.js — generic KV document store over HTTP + WebSocket.
 // firebase mirror: doc-tree model. Thin REST, path-based WS push.
 // Adapter (src/storage/server.js) = passthrough, no shape translation.
+// TEST: cache layer rebuild check.
 
 'use strict';