feat(06-01): migrate categories from emoji to Lucide icon field

- Rename emoji column to icon in schema, Zod schemas, and all services
- Add Drizzle migration with emoji-to-icon data conversion
- Update test helper, seed, and all test files for icon field
- All 87 tests pass with new icon-based schema

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/server/services/thread.service.ts

@@ -22,7 +22,7 @@ export function getAllThreads(db: Db = prodDb, includeResolved = false) {
       resolvedCandidateId: threads.resolvedCandidateId,
       categoryId: threads.categoryId,
       categoryName: categories.name,
-      categoryEmoji: categories.emoji,
+      categoryIcon: categories.icon,
       createdAt: threads.createdAt,
       updatedAt: threads.updatedAt,
       candidateCount: sql<number>`(
@@ -67,7 +67,7 @@ export function getThreadWithCandidates(db: Db = prodDb, threadId: number) {
       createdAt: threadCandidates.createdAt,
       updatedAt: threadCandidates.updatedAt,
       categoryName: categories.name,
-      categoryEmoji: categories.emoji,
+      categoryIcon: categories.icon,
     })
     .from(threadCandidates)
     .innerJoin(categories, eq(threadCandidates.categoryId, categories.id))