Fix AI suggestions not displaying after job completion

BREAKING CHANGE: AI assignment job now stores suggestions in database

- Add suggestionsJson column to AssignmentJob table
- Store enriched suggestions when job completes
- Update getAISuggestions to retrieve stored suggestions instead of regenerating
- Filter out already-assigned pairs from stored suggestions

Previously, the background job generated suggestions but discarded them,
and getAISuggestions tried to regenerate from scratch (causing infinite loading).

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

prisma/schema.prisma

@@ -1105,6 +1105,7 @@ model AssignmentJob {
   currentBatch     Int               @default(0)
   processedCount   Int               @default(0)
   suggestionsCount Int               @default(0)
+  suggestionsJson  Json?             @db.JsonB  // Stores the AI-generated suggestions
   errorMessage     String?           @db.Text
   startedAt        DateTime?
   completedAt      DateTime?