feat(server,modules): bootstrap server and module framework

Implements Phases 02 (partial) and 03 of the go-port-cloud-run plan.
Introduces module framework with per-module KV prefix isolation,
health check endpoint, request timeout protection, and comprehensive
test coverage. Cloud Run deployment deferred to Phase 01.

Security hardening: constant-time secret comparison, cron auth bridge,
and secrets stripped from dependency environment exports. Includes
Dockerfile, GitHub CI workflow (vet + race + build), and integration
tests for module lifecycle.

.github/workflows/ci.yml

@@ -0,0 +1,33 @@
+name: ci
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  go:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        go: ['1.23']
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: ${{ matrix.go }}
+          cache: true
+
+      - name: go vet
+        run: go vet ./...
+
+      - name: go test
+        run: go test -race -count=1 ./...
+
+      - name: go build
+        run: go build ./...