feat: task comments/discussion system

- New task_comments table (separate from progress notes)
- Backend: GET/POST/DELETE /api/tasks/:id/comments with session + bearer auth
- TaskComments component on TaskPage (full-page view) with markdown support,
  author avatars, delete own comments, 30s polling
- CompactComments in TaskDetailPanel (side panel) with last 3 + expand
- Comment API functions in frontend lib/api.ts

backend/src/db/schema.ts

@@ -103,6 +103,20 @@ export const tasks = pgTable("tasks", {
 export type Task = typeof tasks.$inferSelect;
 export type NewTask = typeof tasks.$inferInsert;
 
+// ─── Comments ───
+
+export const taskComments = pgTable("task_comments", {
+  id: uuid("id").defaultRandom().primaryKey(),
+  taskId: uuid("task_id").notNull().references(() => tasks.id, { onDelete: "cascade" }),
+  authorId: text("author_id"), // BetterAuth user ID, or "hammer" for API, null for anonymous
+  authorName: text("author_name").notNull(),
+  content: text("content").notNull(),
+  createdAt: timestamp("created_at", { withTimezone: true }).defaultNow().notNull(),
+});
+
+export type TaskComment = typeof taskComments.$inferSelect;
+export type NewTaskComment = typeof taskComments.$inferInsert;
+
 // ─── BetterAuth tables ───
 
 export const users = pgTable("users", {