Add comprehensive E2E testing infrastructure including:
docker-compose.playwright.yml for test environment orchestration
TestDataManager utility for per-test namespace isolation
Wait helpers for flaky test prevention
Role-based auth fixtures for admin/user/guest testing
GitHub Actions e2e-tests.yml with 4-shard parallelization
Health check utility for service readiness validation
Phase 0 of 10-week E2E testing plan (Supervisor approved 9.2/10)
All 52 existing E2E tests pass with new infrastructure
