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/i18n/locales/en/translation.json

@@ -669,12 +669,17 @@
       "priorityLabel": "Priority",
       "statusLabel": "Status",
       "cancel": "Cancel",
-      "add": "Add task"
+      "add": "Add task",
+      "assigneeDepartment": "Department",
+      "assigneePerson": "Person",
+      "person": "Team member",
+      "personEmpty": "No clinicians available to assign."
     },
     "list": {
       "forDept": "For {{dept}}",
       "personal": "Personal",
-      "byCreator": "by {{name}}"
+      "byCreator": "by {{name}}",
+      "forPerson": "For {{name}}"
     },
     "detail": {
       "fallbackTitle": "Task",
@@ -710,7 +715,9 @@
       "addFailedTitle": "Couldn't add task",
       "addFailedBody": "Please try again.",
       "updateFailedTitle": "Couldn't update task",
-      "updateFailedBody": "Please try again."
+      "updateFailedBody": "Please try again.",
+      "needPersonTitle": "Pick a team member",
+      "needPersonBody": "Choose who this task is for."
     }
   },
   "meetings": {