feat(P7.1): remove task_type='task' — migrate to issue/defect, update defaults and DB migration

app/api/routers/misc.py

@@ -445,7 +445,7 @@ def create_milestone_task(project_code: str, milestone_id: int, task_data: dict,
         description=task_data.get("description"),
         status=TaskStatus.OPEN,
         priority=TaskPriority.MEDIUM,
-        task_type=task_data.get("task_type", "task"),
+        task_type=task_data.get("task_type", "issue"),  # P7.1: default changed from 'task' to 'issue'
         task_subtype=task_data.get("task_subtype"),
         project_id=project.id,
         milestone_id=milestone_id,

app/api/routers/tasks.py

@@ -46,7 +46,7 @@ TASK_SUBTYPE_MAP = {
     'story': {'feature', 'improvement', 'refactor'},
     'test': {'regression', 'security', 'smoke', 'stress'},
     'research': set(),
-    'task': {'defect'},
+    # P7.1: 'task' type removed — defect subtype migrated to issue/defect
     'resolution': set(),
 }
 ALLOWED_TASK_TYPES = set(TASK_SUBTYPE_MAP.keys())