feat(tasks): add code field to BacklogTask type and all task selects

Adds `code: string | null` to BacklogTask interface and includes it in
all Prisma task.findMany selects (backlog API, stories tasks API, page
hydration routes). Updates coerceTaskPayload and test fixtures to match.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/api/products/[id]/backlog/route.ts

@@ -66,6 +66,7 @@ export async function GET(
       orderBy: [{ sort_order: 'asc' }, { created_at: 'asc' }],
       select: {
         id: true,
+        code: true,
         title: true,
         description: true,
         priority: true,

app/api/stories/[id]/tasks/route.ts

@@ -33,6 +33,7 @@ export async function GET(
     orderBy: [{ sort_order: 'asc' }, { created_at: 'asc' }],
     select: {
       id: true,
+      code: true,
       title: true,
       description: true,
       priority: true,