E2E web CI follow-ups from PR #368 review

[ZappoMan]

Follow-up items from Sarah & Mei reviews on PR #368 that are non-blocking but worth tracking.

CI / workflow

• Playwright browser cache: on cache hit, skip playwright install --with-deps; use install-deps only when deps are missing
• PLAYWRIGHT_OUTCOME: replace workflow expression with an explicit step that sets outcome from job results
• Serial job --project order: add a workflow comment that project order is load-bearing for shared-state / signup-mode isolation
• Preview warm-up curl: log a warning instead of silently swallowing failures (curl || true)
• PLAYWRIGHT_WORKERS: consider reducing from 4 → 2 on 2-vCPU runners if flakes persist

Test fixtures / specs

• Admin waitlist invite cleanup: use waitForWaitlistEntry in finally instead of bare findWaitlistEntryByEmail for eventual consistency
• withSignupMode default: document or verify assumption that null waitlist mode resolves to 'open'
• Stripe checkout probe side effect: probeStripeCheckoutReady creates a real Stripe session — consider a health endpoint or document the side effect in TESTING.md

Reporting / unit tests

• mergePlaywrightJsonReports: add unit test for merging errors arrays across shard JSON reports
• Category summary table: surface passedAfterRetry as its own column instead of folding into passed count

Context

• Parent PR: Expand Playwright E2E coverage and category-grouped CI reports #368
• Blocking items from the same review were addressed in commit 05f4572 (chromium auth verify, waitlist edge runtime probe, Stripe beforeAll try/catch).