feat(test): add Phase 5b integration and E2E tests

Add comprehensive integration and end-to-end test suites:
- Integration tests for API flows (authorization, token, verification)
- Integration tests for middleware chain and security headers
- Integration tests for domain verification services
- E2E tests for complete authentication flows
- E2E tests for error scenarios and edge cases
- Shared test fixtures and utilities in conftest.py
- Rename Dockerfile to Containerfile for Podman compatibility

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

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

pyproject.toml

@@ -130,6 +130,7 @@ omit = [
 precision = 2
 show_missing = true
 skip_covered = false
+fail_under = 80
 exclude_lines = [
     "pragma: no cover",
     "def __repr__",