feat: customer relationship view, idempotency keys, approval decline, seed payments, backlog cleanup

- #3: Wire ApprovalModal handleDecline to POST /api/admin/bookings/{id}/cancel
- #25: New GET /api/admin/users/{id}/relationship endpoint with spend, tips, visits, customer-for duration, top services
- #25: UserModal reorganized — Personal Info, Booking History, Customer Relationship, Loyalty, Patch Tests
- #36: Idempotency keys on user and admin booking creation (UUID header, duplicate detection)
- local-dev-2.sh: seed payments via PL/pgSQL for completed bookings (5 randomized scenarios)
- local-dev-2.sh: shrink guest/time-blocker output, add payments to summary
- Backlog: mark #3/#25/#35/#36/#49 done, plan #36/#45, remove #46/#48, update #45 with milestone campaigns
- Remove notes history table, avg visits/year metric, Account Information, Privacy & Consent from UserModal

backend/main.go

@@ -246,12 +246,13 @@ func main() {
 				r.Post("/{id}/edit-requests/{request_id}/deny", bookings.AdminRejectEditRequestHandler)
 			})
 
-			r.Route("/admin/users", func(r chi.Router) {
-				r.Get("/", user.ListAdminUsersHandler)
-				r.Get("/{id}", user.GetAdminUserHandler)
-				r.Get("/{id}/patch-tests/eligible", user.GetEligiblePatchTestServicesHandler)
-				r.Post("/{id}/patch-tests", user.AddPatchTestHandler)
-			})
+r.Route("/admin/users", func(r chi.Router) {
+			r.Get("/", user.ListAdminUsersHandler)
+			r.Get("/{id}", user.GetAdminUserHandler)
+			r.Get("/{id}/relationship", user.GetCustomerRelationshipHandler)
+			r.Get("/{id}/patch-tests/eligible", user.GetEligiblePatchTestServicesHandler)
+			r.Post("/{id}/patch-tests", user.AddPatchTestHandler)
+		})
 
 			r.Route("/admin/today", func(r chi.Router) {
 				r.Get("/current-next", today.GetCurrentAndNextHandler)