ci: add GitHub Actions workflow for test suite

- Added test.yml workflow to run on PRs and pushes to main/master
- Runs server tests with coverage on every PR
- Uploads coverage reports to Codecov
- Added test:server:coverage script to root package.json
- Coverage thresholds enforced: 80% lines/functions, 75% branches

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

package.json

@@ -24,6 +24,7 @@
     "test": "npm run test --workspace=apps/app",
     "test:headed": "npm run test:headed --workspace=apps/app",
     "test:server": "npm run test --workspace=apps/server",
+    "test:server:coverage": "npm run test:cov --workspace=apps/server",
     "dev:marketing": "npm run dev --workspace=apps/marketing"
   }
 }