Manual E2E Test

Test a PR/branch end-to-end by building the full platform, interacting via browser and API, capturing screenshots, and reporting results.

Changelog 2.2.0 — auth flow updated for Better Auth (Supabase signup is gone), env-setup gaps closed, a proven Playwright fallback for agent-browser, a billing-test trap that produces false passes, safer process cleanup, and a mock-provider pattern for deterministic $0 testing. Learned on #14206 — see the evidence comment. 2.2.1 — corrects two claims from 2.2.0 that didn't survive live-stack verification (JWKS does not rotate on a frontend restart; the local Postgres port is 5432, not 54322) and hardens the auth setup (explicit password-length/allowlist/rate-limit failure modes, fail-fast on an empty token, password kept out of process args). 2.3.0 — screenshots are posted as GitHub comment attachments (gh pr comment --attach, needs gh >= 2.99.0) instead of being pushed to a test-screenshots/* branch, which accumulated one branch per tested PR and made every old report's images depend on that branch surviving.