feat: add scheduled sync daemon and TRUNCATE migration mode

- Add sync.py for LAN file sync with atomic writes and mtime-based change detection
- Add watch.py daemon that periodically syncs files and migrates only changed tables
- Add TRUNCATE mode to migrate.py (--truncate flag) to preserve table structure
- Update config.yaml schema with check_interval_minutes and lan_sources
- Update README with new features documentation

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

migrate.py

@@ -216,6 +216,62 @@ def infer_sql_type(col_name, type_rules, primary_key):
     return type_rules.get('default_type', 'NVARCHAR(500)')
 
 
+def table_exists(cursor, schema, table_name):
+    """检查表是否已存在"""
+    full_name = f'[{schema}].[{table_name}]'
+    cursor.execute(f"SELECT OBJECT_ID('{full_name}', 'U')")
+    row = cursor.fetchone()
+    return row[0] is not None
+
+
+def get_existing_columns(cursor, schema, table_name):
+    """查询现有表的列名集合（排除 id 自增列）"""
+    full_name = f'[{schema}].[{table_name}]'
+    cursor.execute(f"""
+        SELECT COLUMN_NAME
+        FROM INFORMATION_SCHEMA.COLUMNS
+        WHERE TABLE_SCHEMA = ? AND TABLE_NAME = ?
+        ORDER BY ORDINAL_POSITION
+    """, (schema, table_name))
+    cols = {row[0] for row in cursor.fetchall()}
+    cols.discard('id')
+    return cols
+
+
+def ensure_table(cursor, table_name, headers, cfg, actual_pk, use_truncate=False):
+    """确保目标表就绪。
+    - use_truncate=False（默认）→ 调用 create_table()，DROP + CREATE
+    - use_truncate=True →
+        1. 表不存在 → CREATE TABLE
+        2. 表存在且列名集合一致 → TRUNCATE TABLE
+        3. 表存在但列名有变化 → DROP + CREATE（兜底）
+    """
+    if not use_truncate:
+        return create_table(cursor, table_name, headers, cfg, actual_pk)
+
+    schema = cfg['schema']
+    full_name = f'[{schema}].[{table_name}]'
+
+    if not table_exists(cursor, schema, table_name):
+        # 表不存在，走正常 CREATE
+        return create_table(cursor, table_name, headers, cfg, actual_pk)
+
+    # 比较列名集合
+    existing_cols = get_existing_columns(cursor, schema, table_name)
+    new_cols = set(headers)
+    # 主键列也应包含在比较中
+    new_cols.add(actual_pk)
+
+    if existing_cols == new_cols:
+        # 列名一致，TRUNCATE
+        cursor.execute(f"TRUNCATE TABLE {full_name}")
+        cursor.commit()
+        return f"TRUNCATE TABLE {full_name}"
+    else:
+        # 列名有变化，兜底 DROP + CREATE
+        return create_table(cursor, table_name, headers, cfg, actual_pk)
+
+
 def create_table(cursor, table_name, headers, cfg, actual_pk):
     """动态创建表。如已存在则先 DROP。"""
     schema = cfg['schema']
@@ -366,7 +422,7 @@ def batch_insert(cursor, table_name, headers, data_dict, cfg, actual_pk):
 #  主流程
 # ============================================================
 
-def migrate_table(cursor, table_config, cfg, dry_run=False):
+def migrate_table(cursor, table_config, cfg, dry_run=False, use_truncate=False):
     """迁移单个表"""
     table_name = table_config['table']
     sources = table_config['sources']
@@ -400,8 +456,9 @@ def migrate_table(cursor, table_config, cfg, dry_run=False):
         return len(merged)
 
     # 2. 建表
-    create_sql = create_table(cursor, table_name, headers, cfg, actual_pk)
-    print(f"    建表: [{cfg['schema']}].[{table_name}] ({len(headers)} 列)")
+    create_sql = ensure_table(cursor, table_name, headers, cfg, actual_pk, use_truncate=use_truncate)
+    action = "TRUNCATE" if use_truncate and "TRUNCATE" in (create_sql or "") else "建表"
+    print(f"    {action}: [{cfg['schema']}].[{table_name}] ({len(headers)} 列)")
 
     # 3. 插入数据
     total = batch_insert(cursor, table_name, headers, merged, cfg, actual_pk)
@@ -414,6 +471,8 @@ def main():
     parser = argparse.ArgumentParser(description='Excel to SQL Server Migration')
     parser.add_argument('--config', default=None, help='配置文件路径 (默认: config.yaml)')
     parser.add_argument('--dry-run', action='store_true', help='仅预览，不写入数据库')
+    parser.add_argument('--truncate', action='store_true',
+                        help='使用 TRUNCATE 模式（表存在时截断而非重建）')
     parser.add_argument('--table', nargs='+', default=None,
                         help='只迁移指定的表，例如 --table PG_2022 TM_2025')
     args = parser.parse_args()
@@ -455,7 +514,8 @@ def main():
     total_rows = 0
     for i, table_cfg in enumerate(tables, 1):
         print(f"\n[{i}/{total_tables}]", end='')
-        rows = migrate_table(cursor, table_cfg, cfg, dry_run=args.dry_run)
+        rows = migrate_table(cursor, table_cfg, cfg, dry_run=args.dry_run,
+                             use_truncate=args.truncate)
         total_rows += rows
 
     if not args.dry_run: