feat: add 'cancelled' task status; fix 500 on invalid status/priority

TaskStatus enum was missing 'cancelled' — the LLM tried to use it and
hit TaskStatus("cancelled") raising ValueError → 500. Added the value,
a migration to extend the task_status Postgres enum, and proper 400
validation guards on both create and update task routes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/models/note.py

@@ -13,6 +13,7 @@ class TaskStatus(str, enum.Enum):
     todo = "todo"
     in_progress = "in_progress"
     done = "done"
+    cancelled = "cancelled"
 
 
 class TaskPriority(str, enum.Enum):