Tasks: optional persona for LLM + research tasks (biases output voice)

Wire the existing built-in PERSONAS catalog through to scheduled tasks
the same way I wired it to reminder synthesis. Repurposes the
dormant scheduled_tasks.character_id column.

UI (static/js/tasks.js)
- New 'Persona' select in the LLM / Research task form, with the five
  built-in characters (socrates/razor/nietzsche/spark/odysseus) plus a
  default 'no persona' option. Pre-populates from existing.character_id
  on edit. Non-llm/research types explicitly clear it on save.

API (routes/task_routes.py)
- TaskCreate + TaskUpdate gain character_id: Optional[str].
- _task_to_dict echoes character_id back so the form can hydrate on
  edit. Update endpoint stores '' as None to allow clearing.

Runner (src/task_scheduler.py)
- When task.character_id is set and matches a built-in persona, prepend
  the persona prompt to the task system prompt so the model speaks in
  that voice while still knowing it's running a scheduled task.
- crew_member.personality still wins as the base; character_id stacks
  on top.

routes/task_routes.py

@@ -151,6 +151,7 @@ class TaskCreate(BaseModel):
     endpoint_url: Optional[str] = None
     then_task_id: Optional[str] = None            # chain: run this task after success
     notifications_enabled: Optional[bool] = None  # None lets action-specific defaults apply
+    character_id: Optional[str] = None             # built-in persona id (PERSONAS) — biases output voice
 
 
 class TaskUpdate(BaseModel):
@@ -171,6 +172,7 @@ class TaskUpdate(BaseModel):
     endpoint_url: Optional[str] = None
     then_task_id: Optional[str] = None
     notifications_enabled: Optional[bool] = None
+    character_id: Optional[str] = None
 
 
 def _display_task_name(t: ScheduledTask) -> str:
@@ -203,6 +205,7 @@ def _task_to_dict(t: ScheduledTask, include_last_run_result: bool = False) -> di
         "output_target": t.output_target,
         "session_id": t.session_id,
         "crew_member_id": getattr(t, "crew_member_id", None),
+        "character_id": getattr(t, "character_id", None),
         "model": t.model,
         "endpoint_url": t.endpoint_url,
         "run_count": t.run_count or 0,
@@ -552,6 +555,7 @@ def setup_task_routes(task_scheduler) -> APIRouter:
                 then_task_id=then_task_id,
                 webhook_token=webhook_token,
                 notifications_enabled=notifications_enabled,
+                character_id=(req.character_id or None),
             )
             db.add(task)
             db.commit()
@@ -705,6 +709,9 @@ def setup_task_routes(task_scheduler) -> APIRouter:
                 task.then_task_id = _validate_then_task_id(db, req.then_task_id, user, current_task_id=task.id)
             if req.notifications_enabled is not None:
                 task.notifications_enabled = bool(req.notifications_enabled)
+            if req.character_id is not None:
+                # Empty string clears the persona; non-empty stores the id.
+                task.character_id = req.character_id or None
             if req.cron_expression is not None:
                 if req.cron_expression:
                     try: