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/drizzle/meta/_journal.json

@@ -183,6 +183,13 @@
       "when": 1781910033543,
       "tag": "0025_nice_paladin",
       "breakpoints": true
+    },
+    {
+      "idx": 26,
+      "version": "7",
+      "when": 1781969782874,
+      "tag": "0026_many_wolfsbane",
+      "breakpoints": true
     }
   ]
 }