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

frontend/src/lib/components/admin/WalkInCreateModal.svelte

@@ -67,6 +67,7 @@
 	>({});
 
 	let submitting = $state(false);
+	let idempotencyKey = $state<string>('');
 
 	// Countdown state
 	let reservationCountdown = $state<string>('');
@@ -262,6 +263,11 @@
 		submitting = true;
 
 		try {
+			// Generate idempotency key if not already set (reused on retry)
+			if (!idempotencyKey) {
+				idempotencyKey = crypto.randomUUID();
+			}
+
 			// Validate duration doesn't exceed available slot
 			if (maxSlotDuration > 0 && getTotalDuration() > maxSlotDuration) {
 				toast.error(
@@ -358,6 +364,7 @@
 				method: 'POST',
 				headers: {
 					'Content-Type': 'application/json',
+					'Idempotency-Key': idempotencyKey,
 					Authorization: `Bearer ${authStore.currentToken}`
 				},
 				body: JSON.stringify(payload)