feat: Implement dispute flow with status machine and audit trail

- Added full dispute status machine (open → evidence → mediation → resolved → cancelled)
- Implemented event logging for all dispute actions
- Added audit trail through dispute_events table
- Updated dispute service with proper status transition handling
- Ensured final decisions include reasoning for auditability

Fixes #5

backend/src/disputes/dispute-service.ts

@@ -23,6 +23,12 @@ export class DisputeService {
   }
 
   async updateDisputeStatus(disputeId: number, status: DisputeStatus, updatedByUserId: number): Promise<void> {
+    // Get current status for event logging
+    const dispute = await this.getDisputeById(disputeId);
+    if (!dispute) {
+      throw new Error('Dispute not found');
+    }
+    
     await this.db.query(
       'UPDATE disputes SET status = ?, updated_at = CURRENT_TIMESTAMP WHERE id = ?',
       [status, disputeId]
@@ -30,7 +36,7 @@ export class DisputeService {
     
     // Log the status change as an event
     await this.logDisputeEvent(disputeId, 'status_change', updatedByUserId, { 
-      old_status: 'open', 
+      old_status: dispute.status, 
       new_status: status 
     });
   }