build: move Docker base image to Node 22

Dockerfile

@@ -1,7 +1,7 @@
 # Multi-stage: esbuild produces a single Node file (deps inlined) + minified UI.
 # Final image has no node_modules — only dist/ + runtime Node.
 
-FROM node:20-alpine AS build
+FROM node:22-alpine@sha256:fe4f6d74e20370acb04ad889077d14d0f64a9c8ea6a42bc60cddf1e844699914 AS build
 WORKDIR /app
 COPY package.json package-lock.json ./
 RUN npm ci
@@ -10,7 +10,7 @@ COPY ui/ ./ui/
 COPY scripts/ ./scripts/
 RUN npm run build
 
-FROM node:20-alpine
+FROM node:22-alpine@sha256:fe4f6d74e20370acb04ad889077d14d0f64a9c8ea6a42bc60cddf1e844699914
 WORKDIR /app
 ENV NODE_ENV=production
 # Runtime is only the bundled server + UI (no node_modules, no source, no mcp-only entry)