tasks: assign a task to a specific person

Backend: add tasks.assignee_user_id (nullable, fk user) + migration; persist
it through create/update and surface tasks to the assigned person in listTasks.

Frontend: New Task dialog gains a three-way Assignee control
(Myself / Department / Person) with a provider picker from /api/staff/providers;
task card + detail show the assignee's name when set.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/tasks.ts

@@ -12,6 +12,8 @@ export type Task = {
   assignee: string;
   // Department (member role) the task is assigned to; null = personal task.
   assigneeRole: string | null;
+  // A specific person the task is assigned to (user id); null otherwise.
+  assigneeUserId: string | null;
   due: string;
   priority: Priority;
   status: TaskStatus;
@@ -29,6 +31,7 @@ export type TaskInput = {
   title: string;
   assignee?: string;
   assigneeRole?: string | null;
+  assigneeUserId?: string | null;
   due?: string;
   priority?: Priority;
   status?: TaskStatus;