Phase 1: Financial audit logging foundation

- Add FinancialAuditLog model to track all financial changes
- Log action (CREATE, UPDATE, DELETE) with before/after values
- Capture who made changes (IP address)
- Record timestamp and reason for change
- Create /admin/financial-audit page to view audit trail
- Add filtering by entity type, action, date range
- Show summary stats (total changes, deletions, etc)
- Integrate with purchases: log create and delete operations
- Red-flag deletions for security awareness
- Add "Financial audit trail" link to admin menu

This provides foundation for Phase 2 (reporting UI) and Phase 3 (bank reconciliation).
All financial changes are now traceable and auditable for compliance.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

prisma/schema.prisma

@@ -404,3 +404,16 @@ model CustomerLoginLog {
   userAgent String?
   success   Boolean  @default(true) // false for failed login attempts
 }
+
+// Audit log for financial transactions — tracks all changes to financial data
+model FinancialAuditLog {
+  id          String   @id @default(cuid())
+  action      String   // CREATE, UPDATE, DELETE
+  entityType  String   // "ORDER", "MANUAL_SALE", "PURCHASE", "EXPENSE"
+  entityId    String   // ID of the order/sale/purchase/expense
+  changes     String   // JSON: {field: {old: X, new: Y}, ...}
+  amount      Int?     // cents, for quick filtering of high-value changes
+  changedAt   DateTime @default(now())
+  changedBy   String?  // "SYSTEM" or IP address if admin
+  reason      String?  // why was this changed
+}