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

app/main.py

@@ -129,7 +129,7 @@ def _migrate_schema():
         # tasks extra fields
         result = db.execute(text("SHOW COLUMNS FROM tasks LIKE 'task_type'"))
         if not result.fetchone():
-            db.execute(text("ALTER TABLE tasks ADD COLUMN task_type VARCHAR(32) DEFAULT 'task'"))
+            db.execute(text("ALTER TABLE tasks ADD COLUMN task_type VARCHAR(32) DEFAULT 'issue'"))
         result = db.execute(text("SHOW COLUMNS FROM tasks LIKE 'task_subtype'"))
         if not result.fetchone():
             db.execute(text("ALTER TABLE tasks ADD COLUMN task_subtype VARCHAR(64) NULL"))
@@ -194,6 +194,10 @@ def _migrate_schema():
             if not _has_column(db, "milestones", "started_at"):
                 db.execute(text("ALTER TABLE milestones ADD COLUMN started_at DATETIME NULL"))
 
+        # --- P7.1: Migrate task_type='task' to 'issue' ---
+        if _has_table(db, "tasks") and _has_column(db, "tasks", "task_type"):
+            db.execute(text("UPDATE tasks SET task_type='issue' WHERE task_type='task'"))
+
         # --- Task status enum migration (old -> new) ---
         if _has_table(db, "tasks"):
             # Widen enum first