feat: Add idempotency to useModerationActions

Implement idempotency integration in the useModerationActions hook as per the detailed plan.

src/lib/idempotencyHelpers.ts

@@ -0,0 +1,90 @@
+/**
+ * Idempotency Key Utilities
+ * 
+ * Provides helper functions for generating and managing idempotency keys
+ * for moderation operations to prevent duplicate requests.
+ */
+
+/**
+ * Generate a deterministic idempotency key for a moderation action
+ * 
+ * Format: action_submissionId_itemIds_userId_timestamp
+ * Example: approval_abc123_def456_ghi789_user123_1699564800000
+ * 
+ * @param action - The moderation action type ('approval', 'rejection', 'retry')
+ * @param submissionId - The submission ID
+ * @param itemIds - Array of item IDs being processed
+ * @param userId - The moderator's user ID
+ * @returns Deterministic idempotency key
+ */
+export function generateIdempotencyKey(
+  action: 'approval' | 'rejection' | 'retry',
+  submissionId: string,
+  itemIds: string[],
+  userId: string
+): string {
+  // Sort itemIds to ensure consistency regardless of order
+  const sortedItemIds = [...itemIds].sort().join('_');
+  
+  // Include timestamp to allow same moderator to retry after 24h window
+  const timestamp = Date.now();
+  
+  return `${action}_${submissionId}_${sortedItemIds}_${userId}_${timestamp}`;
+}
+
+/**
+ * Check if an error is a 409 Conflict (duplicate request)
+ * 
+ * @param error - Error object to check
+ * @returns True if error is 409 Conflict
+ */
+export function is409Conflict(error: unknown): boolean {
+  if (!error || typeof error !== 'object') return false;
+  
+  const errorObj = error as { status?: number; message?: string };
+  
+  // Check status code
+  if (errorObj.status === 409) return true;
+  
+  // Check error message for conflict indicators
+  const message = errorObj.message?.toLowerCase() || '';
+  return message.includes('duplicate request') || 
+         message.includes('already in progress') ||
+         message.includes('race condition');
+}
+
+/**
+ * Extract retry-after value from error response
+ * 
+ * @param error - Error object with potential Retry-After header
+ * @returns Seconds to wait before retry, defaults to 3
+ */
+export function getRetryAfter(error: unknown): number {
+  if (!error || typeof error !== 'object') return 3;
+  
+  const errorObj = error as { 
+    retryAfter?: number;
+    context?: { headers?: { 'Retry-After'?: string } }
+  };
+  
+  // Check structured retryAfter field
+  if (errorObj.retryAfter) return errorObj.retryAfter;
+  
+  // Check Retry-After header
+  const retryAfterHeader = errorObj.context?.headers?.['Retry-After'];
+  if (retryAfterHeader) {
+    const seconds = parseInt(retryAfterHeader, 10);
+    return isNaN(seconds) ? 3 : seconds;
+  }
+  
+  return 3; // Default 3 seconds
+}
+
+/**
+ * Sleep for a specified duration
+ * 
+ * @param ms - Milliseconds to sleep
+ */
+export function sleep(ms: number): Promise<void> {
+  return new Promise(resolve => setTimeout(resolve, ms));
+}