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>

backend/src/lib/task-validation.ts

@@ -15,6 +15,7 @@ export const taskInputSchema = z.object({
   title: z.string().trim().min(1, "A task subject is required.").max(200),
   assignee: z.string().trim().max(200).default("Unassigned"),
   assigneeRole: z.enum(TASK_DEPARTMENTS).nullish(),
+  assigneeUserId: z.string().trim().max(120).nullish(),
   due: z.string().trim().max(120).default("No due date"),
   priority: z.enum(["high", "medium", "low"]).default("medium"),
   status: z.enum(["todo", "in_progress", "done"]).default("todo"),