feat(backend): add automatic DB migrations

Add a lightweight migration runner with schema_migrations tracking, run pending migrations during backend startup before the scheduler, and keep a manual backend-migrate entrypoint.

The change also moves the existing lockout and task-thread-ID schema steps into shared migration modules, updates docs, and archives the OpenSpec change.

apps/backend/scripts/run_migrations.py

@@ -0,0 +1,33 @@
+#!/usr/bin/env python3
+"""
+运行数据库迁移的脚本。
+
+使用方法:
+    uv run python -m backend.scripts.run_migrations
+"""
+
+from __future__ import annotations
+
+import logging
+
+from backend.migrations import MigrationExecutionError, run_pending_migrations
+from backend.models import init_db
+
+logging.basicConfig(level=logging.INFO)
+logger = logging.getLogger(__name__)
+
+
+def main() -> int:
+    try:
+        init_db()
+        result = run_pending_migrations()
+    except MigrationExecutionError as exc:
+        logger.error("❌ 迁移失败: %s", exc)
+        return 1
+
+    logger.info("✅ 迁移完成：applied=%s skipped=%s", len(result.applied), len(result.skipped))
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())