feat: sequential task IDs (HQ-1, HQ-2, etc.)

- Add serial task_number column to tasks table
- Display HQ-{number} on cards and detail panel
- API resolveTask() supports UUID, number, or HQ-N prefix
- GET /api/tasks/:id endpoint for single task lookup
- All PATCH/POST/DELETE endpoints resolve by number or UUID

frontend/src/lib/types.ts

@@ -9,6 +9,7 @@ export interface ProgressNote {
 
 export interface Task {
   id: string;
+  taskNumber: number;
   title: string;
   description: string | null;
   source: TaskSource;