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

frontend/src/lib/api.ts

@@ -167,6 +167,42 @@ export async function addProgressNote(taskId: string, note: string): Promise<Tas
   return res.json();
 }
 
+// ─── Comments API ───
+
+export interface TaskComment {
+  id: string;
+  taskId: string;
+  authorId: string | null;
+  authorName: string;
+  content: string;
+  createdAt: string;
+}
+
+export async function fetchComments(taskId: string): Promise<TaskComment[]> {
+  const res = await fetch(`${BASE}/${taskId}/comments`, { credentials: "include" });
+  if (!res.ok) throw new Error("Failed to fetch comments");
+  return res.json();
+}
+
+export async function addComment(taskId: string, content: string): Promise<TaskComment> {
+  const res = await fetch(`${BASE}/${taskId}/comments`, {
+    method: "POST",
+    credentials: "include",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify({ content }),
+  });
+  if (!res.ok) throw new Error("Failed to add comment");
+  return res.json();
+}
+
+export async function deleteComment(taskId: string, commentId: string): Promise<void> {
+  const res = await fetch(`${BASE}/${taskId}/comments/${commentId}`, {
+    method: "DELETE",
+    credentials: "include",
+  });
+  if (!res.ok) throw new Error("Failed to delete comment");
+}
+
 // Admin API
 export async function fetchUsers(): Promise<any[]> {
   const res = await fetch("/api/admin/users", { credentials: "include" });