Give every task a bot: drop shared workspace tasks

A workspace task with no bot was seen by every bot working there. Now a
task can't be saved without a bot, a worker reaches only its own (the
workspace admin and the coordinator still reach all), the workspace page
picks who has a new task, and a bot leaving a workspace or deleted passes
its tasks there to the workspace's admin bot, else the coordinator (also
at boot for rows without a bot).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/functions/backend/db/tasks/move-task.ts

@@ -10,7 +10,7 @@ type Params = {
     task_id: number | string;
     /**
      * Where it goes: a bot's own list (`bot_id` alone), or a workspace,
-     * with the bot that has it there (none: unassigned).
+     * with the bot that has it there (always one).
      */
     bot_id?: number | string | null;
     workspace_id?: number | string | null;