ops: integrate E2E tests into CI pipeline

[ericfitz]

Summary

A manual-trigger CI workflow stub for E2E tests has been added (.github/workflows/e2e-tests.yml). This issue tracks making it fully automated.

Work Items

• Backend service provisioning in CI (Docker Compose or GitHub service containers)
• Trigger strategy (on PR, on push to main, on schedule)
• Parallel browser matrix (currently Chromium-only)
• Test result reporting (PR comments, status checks)
• Secrets management for OAuth test credentials
• Test data isolation between CI runs

Context

• Stub workflow: .github/workflows/e2e-tests.yml (workflow_dispatch only)
• Related: test: automated UX integration testing #84 (automated UX integration testing)
• Backend repo: https://github.com/ericfitz/tmi