setup: add GitHub Actions CI pipeline

[Lewiskunta]

Adds a four-job CI pipeline:

• Backend: lint, go mod tidy check, migrations, tests with
  race detection, coverage threshold, server and CLI build
• Frontend: ESLint, TypeScript type check, production build
• Security: Go vulnerability scan, secret detection
• PR title: Conventional Commits format enforcement

Coverage threshold set to 0 during Phase 0. Will be raised as real tests are added.