Optimize AI system with batching, token tracking, and GDPR compliance

- Add AIUsageLog model for persistent token/cost tracking
- Implement batched processing for all AI services:
  - Assignment: 15 projects/batch
  - Filtering: 20 projects/batch
  - Award eligibility: 20 projects/batch
  - Mentor matching: 15 projects/batch
- Create unified error classification (ai-errors.ts)
- Enhance anonymization with comprehensive project data
- Add AI usage dashboard to Settings page
- Add usage stats endpoints to settings router
- Create AI system documentation (5 files)
- Create GDPR compliance documentation (2 files)

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

prisma/schema.prisma

@@ -684,6 +684,46 @@ model AuditLog {
   @@index([timestamp])
 }
 
+// =============================================================================
+// AI USAGE TRACKING
+// =============================================================================
+
+model AIUsageLog {
+  id               String   @id @default(cuid())
+  createdAt        DateTime @default(now())
+
+  // Who/what triggered it
+  userId           String?
+  action           String   // ASSIGNMENT, FILTERING, AWARD_ELIGIBILITY, MENTOR_MATCHING
+  entityType       String?  // Round, Project, Award
+  entityId         String?
+
+  // What was used
+  model            String   // gpt-4o, gpt-4o-mini, o1, etc.
+  promptTokens     Int
+  completionTokens Int
+  totalTokens      Int
+
+  // Cost tracking
+  estimatedCostUsd Decimal? @db.Decimal(10, 6)
+
+  // Request context
+  batchSize        Int?
+  itemsProcessed   Int?
+
+  // Status
+  status           String   // SUCCESS, PARTIAL, ERROR
+  errorMessage     String?
+
+  // Detailed data (optional)
+  detailsJson      Json?    @db.JsonB
+
+  @@index([userId])
+  @@index([action])
+  @@index([createdAt])
+  @@index([model])
+}
+
 // =============================================================================
 // NOTIFICATION LOG (Phase 2)
 // =============================================================================