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/ApprovalModal.svelte

@@ -288,10 +288,34 @@
 	}
 
 	async function handleDecline() {
-		// TODO: Implement decline/cancel endpoint
-		toast.info('Decline booking - Coming soon');
-		showDeclineConfirm = false;
-		open = false;
+		submitting = true;
+		const loadingToast = toast.loading('Declining booking...');
+
+		try {
+			const response = await fetch(`/api/admin/bookings/${booking.id}/cancel`, {
+				method: 'POST',
+				headers: {
+					'Content-Type': 'application/json',
+					Authorization: `Bearer ${authStore.currentToken}`
+				}
+			});
+
+			if (response.ok) {
+				toast.success('Booking declined successfully', { id: loadingToast });
+				showDeclineConfirm = false;
+				open = false;
+				onApproved();
+			} else {
+				const text = await response.text();
+				toast.error('Failed to decline: ' + text, { id: loadingToast });
+			}
+		} catch (err) {
+			console.error('Error declining booking:', err);
+			toast.error('Network error declining booking', { id: loadingToast });
+		} finally {
+			submitting = false;
+			showDeclineConfirm = false;
+		}
 	}
 </script>