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):

src/fabledassistant/routes/tasks.py

@@ -71,10 +71,14 @@ async def create_task_route():
     if isinstance(due_date, tuple):
         return due_date
 
-    status = TaskStatus(data["status"]).value if "status" in data else TaskStatus.todo.value
-    priority = (
-        TaskPriority(data["priority"]).value if "priority" in data else TaskPriority.none.value
-    )
+    try:
+        status = TaskStatus(data["status"]).value if "status" in data else TaskStatus.todo.value
+    except ValueError:
+        return jsonify({"error": f"Invalid status: {data['status']}"}), 400
+    try:
+        priority = TaskPriority(data["priority"]).value if "priority" in data else TaskPriority.none.value
+    except ValueError:
+        return jsonify({"error": f"Invalid priority: {data['priority']}"}), 400
 
     project_id = data.get("project_id")
     if project_id is None and data.get("project"):
@@ -123,9 +127,19 @@ async def update_task_route(task_id: int):
     uid = get_current_user_id()
     data = await request.get_json()
     fields = {}
-    for key in ("title", "status", "priority"):
+    for key in ("title",):
         if key in data:
             fields[key] = data[key]
+    if "status" in data:
+        try:
+            fields["status"] = TaskStatus(data["status"]).value
+        except ValueError:
+            return jsonify({"error": f"Invalid status: {data['status']}"}), 400
+    if "priority" in data:
+        try:
+            fields["priority"] = TaskPriority(data["priority"]).value
+        except ValueError:
+            return jsonify({"error": f"Invalid priority: {data['priority']}"}), 400
 
     # Accept both "body" and "description" (prefer body)
     if "body" in data: