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/migrations/20260205220000_add_suggestions_to_assignment_job/migration.sql

@@ -0,0 +1,11 @@
+-- Add suggestionsJson column to AssignmentJob to store AI-generated suggestions
+
+DO $$
+BEGIN
+    IF NOT EXISTS (
+        SELECT 1 FROM information_schema.columns
+        WHERE table_name = 'AssignmentJob' AND column_name = 'suggestionsJson'
+    ) THEN
+        ALTER TABLE "AssignmentJob" ADD COLUMN "suggestionsJson" JSONB;
+    END IF;
+END $$;