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

backend/src/db/schema.ts

@@ -3,6 +3,7 @@ import {
   uuid,
   text,
   integer,
+  serial,
   timestamp,
   jsonb,
   pgEnum,
@@ -40,6 +41,7 @@ export interface ProgressNote {
 
 export const tasks = pgTable("tasks", {
   id: uuid("id").defaultRandom().primaryKey(),
+  taskNumber: serial("task_number").notNull(),
   title: text("title").notNull(),
   description: text("description"),
   source: taskSourceEnum("source").notNull().default("donovan"),