ci: optimize workflows to cut redundant runs (#465)

* ci: optimize workflows to cut redundant runs and add timeout protection

Skip validation jobs (build, test, lint, Docker, E2E) on push-to-main
since code was already validated on the PR. Skip CI entirely for
release-please PRs (version bump only). Add timeout-minutes to all jobs
to prevent hung runners from burning 360 min (GitHub default). Add npm
caching to the start-app composite action. Add concurrency control to
docker-publish. Group Dependabot updates to batch PRs per ecosystem.

Estimated savings: ~10 min per release cycle, ~30-40 min/day.

* chore: add tsbuildinfo to gitignore

.github/actions/start-app/action.yml

@@ -22,6 +22,11 @@ runs:
       uses: actions/setup-node@v6
       with:
         node-version: '20'
+        cache: 'npm'
+        cache-dependency-path: |
+          package-lock.json
+          backend/package-lock.json
+          frontend/package-lock.json
 
     - name: Install root dependencies (Playwright)
       shell: bash