perf: SQLite WAL + 复合索引 + GZip + scheduler jitter + 构建缓存

Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1
+# syntax=docker/dockerfile:1.4
 # ---- Stage 1: Build frontend ----
 FROM node:20-alpine AS frontend-build
 WORKDIR /frontend
@@ -42,7 +42,9 @@ RUN --mount=type=cache,target=/root/.cache/pip \
       -r requirements.txt
 
 # Playwright Chromium：安装在镜像层中，业务代码变更不会触发重下
-RUN playwright install chromium
+# --mount=type=cache 利用 BuildKit 缓存避免每次构建重下 ~170 MB 浏览器
+RUN --mount=type=cache,target=/root/.cache/ms-playwright,sharing=locked \
+    playwright install chromium
 
 # 源码层：业务代码变更不影响上面所有依赖层
 COPY backend/ .