fix: loop-B adversarial findings — tip-type double-charge, tip-refund capacity, loyalty stamp farming, gate ordering, auth amplification, admin audit log

Loop B restart (money/security/dup-mod adversarial) fixes:
- CRITICAL: CreateTerminalPayment rejects payment_type='tip' (mirrors CreateBookingPayment) — a tip-typed admin charge no longer records the FULL amount as a tip and double-collects (all is-paid computations exclude tip rows)
- HIGH: tip refunds can no longer re-open booking capacity — refunded_total subqueries filter payment_type <> 'tip' (service.go) and RefundPayment rejects tip rows
- MEDIUM: loyalty-stamp farming closed — stamp award once-per-booking via loyalty_stamp_awarded_at column (init-script.sql) + existing same-day guard
- MEDIUM: CreateTipPayment/CreateBookingPayment 2FA gates moved AFTER the idempotency completed-dedup (code consumed only on new money paths; terminal path already correct) — lost-response retries return the completed payment instead of 400
- MEDIUM: replayRescueLowerBoundSkew widened to 5m (DB-clock-skew stranded originals now rescued)
- MEDIUM-1: verifyFamilyAlive DB amplification reduced via 30s bounded family-alive cache; admin route group rate-limited
- MEDIUM-3: admin saved-card charges now write admin_audit_log (handlers.go helper + till); [2FA] log line decoupled from user identity
- LOW-1: logout scoped to the presented token's family (no cross-session kill)
- LOW-2: refresh-reuse grace widened for same-IP replays
- LOW-4: squareEnvironmentMismatch enforced for empty env
- LOW-5: uuid.ts hard-fails on Math.random fallback (crypto.randomUUID)
- Cash/giftcard tip-enabled overflow mirrors the card-terminal carve

26/26 backend packages; 72/72 frontend tests + build; env-docs 41/41.

backend/main.go

@@ -701,10 +701,18 @@ func main() {
 
 		r.With(mw.RequireAuth, mw.RequireVerified, limitBody(uploadBodyLimit)).Post("/user/profile-picture", user.UploadProfilePictureHandler)
 
-		// Admin-only (no rate limit - trusted users with authenticated sessions)
+		// Admin-only. The group now carries a per-IP limiter (300/min) to bound
+		// the per-request DB amplification of the auth path (VerifyToken runs a
+		// JTI-revocation query + a family-alive query per request — MEDIUM-1):
+		// an unthrottled admin surface lets a single compromised admin session
+		// hammer the DB. 300/min is far above any legitimate admin UI usage and
+		// keys per-IP (RateLimitByUser would be a no-op — there is exactly one
+		// admin account, so per-user == global). The trusted-session model is
+		// unchanged; this only bounds amplification.
 		r.Group(func(r chi.Router) {
 			r.Use(mw.RequireAuth)
 			r.Use(mw.RequireAdmin)
+			r.Use(mw.RateLimit(300, time.Minute))
 			r.Use(limitBody(defaultBodyLimit))
 
 			r.Route("/admin/services", func(r chi.Router) {