feat: add playwright e2e workflow

tests/e2e/README.md

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+# Playwright Coverage
+
+This suite is the browser-level guardrail for the holiday booking app.
+
+## Current Coverage
+
+- Homepage shell and content structure
+- Health endpoint
+
+## Planned Coverage
+
+- Property listing and detail flows
+- Availability search and date handling
+- Booking form validation and summary
+- Stripe checkout handoff and return states
+- Confirmation and failure states
+- Admin management flows
+- SEO, accessibility, and responsive behavior
+
+## Rule
+
+Update this suite as each user-facing flow lands. The workflow is separate from deploy and only runs against `develop` and `qa`.
+