fix: copy node_modules from build context instead of running npm install

The server's npm proxy intercepts and silently fails devDependency
installs inside Docker. Bundle node_modules directly from the local
checkout where they are known-good.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sync/Dockerfile

@@ -1,8 +1,8 @@
-# Stage 1: build TypeScript + install all deps
+# Stage 1: build TypeScript — copy pre-installed node_modules from context
 FROM node:20-slim AS builder
 WORKDIR /app
 COPY package.json package-lock.json ./
-RUN NODE_ENV=development npm install --registry https://registry.npmjs.org
+COPY node_modules ./node_modules
 COPY tsconfig.json ./
 COPY src ./src
 RUN ./node_modules/.bin/tsc && npm prune --production