test(tier1): add e2e-install target; clarify limiter docs; label brand sample

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

testing/README.md

@@ -5,6 +5,7 @@ One environment-agnostic suite, two targets.
 ## Tier 1 — local Docker (every PR)
 
 ```sh
+make e2e-install     # one-time: install Playwright + chromium (cold checkout)
 make tier1-up        # build + start: gitea(seeded) + backend + web(nginx) + mailpit
 make e2e             # run Playwright against http://localhost:8080
 make fe-unit         # run Vitest frontend unit tests
@@ -24,7 +25,10 @@ Mailpit, so no real OAuth provider is needed.
 request, so the normal "fresh `tier1-up` then one `e2e`" flow is well clear of
 it. Do not retry `make e2e` more than ~4 times in a 5-minute window against the
 same running stack, or the 6th OTC request will 429. Restarting the backend
-container (or `tier1-down`/`tier1-up`) resets the in-process limiter.
+container (or `tier1-down`/`tier1-up`) resets the in-process limiter. Note:
+this is distinct from the per-email 60s OTC cooldown (`OTC_REQUEST_COOLDOWN_SECONDS`,
+`backend/app/otc.py`); the smoke spec handles that separately by using a unique
+email address per run.
 
 ## Tier 2 — PPE (deploy gate)