fix(docker): use pnpm --filter to build each service in Dockerfiles

Root scripts point to old paths (src/, frontend/) that don't exist in the
services/ monorepo layout. Use pnpm --filter to invoke each service's
own build script (tsc) instead.

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

infra/docker/Dockerfile.backend

@@ -20,7 +20,7 @@ COPY services/backend ./services/backend
 RUN pnpm install --frozen-lockfile
 
 # Build backend
-RUN pnpm run build:server
+RUN pnpm --filter './services/backend' run build
 
 # Expose port
 EXPOSE 3001