fix: adversarial review round — replay-rescue double-charge, discount credit, 2FA/per-IP limits, snapshot encryption, refund reconciliation, VAT, frontend parity, tests+docs

Addresses the adversarial fresh-eyes audit (findings A1-A20) plus review-round fixes:
- CRITICAL A1: replay-by-key rescue cross-checks replayed CreatedAt; ccof blind-fail leaves pending with CRITICAL + notification instead of clawing back
- A2/A3/A4: till idempotency key restored to unconditional hash; tip rejected in CreateBookingPayment; campaign discount now reduces the charged amount (deposit credit)
- A5: admin notifications on blind-fail, manual-refund re-arm, cap-stranded charge-group, webhook FAILED/REJECTED refunds
- A6/A10: BuyGiftCard idempotency user-scoped; gift-card slot scan advances past failed rows
- A7/A14/A15: 2FA user+IP limiter, SNAPSHOT_ENC_KEY startup validation, accurate pepper/log-delivery docs
- A8/A9: snapshot encryption on all write+reuse sites; MPV->SPV effective voucher type (single VAT point)
- A11/A12/A13/A16: amount-aware refund reconciliation; completed-booking refund re-validation; till retry dedup; PaymentWasRefunded on SquareClient interface
- A17/A18/A19/A20: CI runs npm test; confirm_overflow_tip frontend dialog; unknown-event admin notification; mock token redaction
- M7 ConfirmOverflowTip, M9 snapshot encryption, C1 discount ordering regression test
- Frontend vitest framework (41 tests), backend coverage for fixed functions, docs corrected (2,269 tests, SUPPORT_EMAIL tokens, resolution status)

All 25 backend packages pass; frontend 41/41; build + env-docs green.

frontend/vitest.config.ts

@@ -0,0 +1,14 @@
+import { defineConfig } from 'vitest/config';
+
+// Separate from vite.config.ts: SvelteKit warns about a `test` block there, and
+// `vite build`/`vite dev` must never pick up test-only config. Pure-logic tests
+// need no svelte/tailwind plugins — no DOM/jsdom this round.
+export default defineConfig({
+	test: {
+		environment: 'node',
+		include: ['src/**/*.test.ts'],
+		exclude: ['**/node_modules/**', '**/*.svelte.test.ts'],
+		restoreMocks: true,
+		unstubEnvs: true
+	}
+});