feat(04-01): add categoryId to threads schema, Zod schemas, and test helper

- Add categoryId column with FK to categories on threads table
- Update createThreadSchema to require categoryId
- Update updateThreadSchema to allow optional categoryId
- Update test helper CREATE TABLE to include category_id

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

src/shared/schemas.ts

@@ -27,10 +27,12 @@ export const updateCategorySchema = z.object({
 // Thread schemas
 export const createThreadSchema = z.object({
   name: z.string().min(1, "Thread name is required"),
+  categoryId: z.number().int().positive(),
 });
 
 export const updateThreadSchema = z.object({
   name: z.string().min(1).optional(),
+  categoryId: z.number().int().positive().optional(),
 });
 
 // Candidate schemas (same fields as items)