refactor: remove Excel migration pipeline; keep Access-only sync

Drop the Excel (.xlsm) production-execution-card pipeline now that the
project only synchronizes Access databases to SQL Server.

- Delete excel_sync_to_sql.py, migration.py, config/field_mappings.py
- Remove Excel-only config symbols (EXCEL_CONFIGS, MIGRATION_TASKS,
  EXECUTION_CARD_FIELDS, CONTRACT_DATA_*, CACHE_DIR, TEMP_DIR,
  EXCEL_SYNC_* settings) from the config package
- Drop now-unused deps from requirements.txt: pandas, sqlalchemy, openpyxl
- Update .env.example, CLAUDE.md, and the uptime_kuma_utils docstring
- Fix the tube-bending workshop Access table mapping in SYNC_MAPPING
  (source table renamed; old name no longer exists)

The three Excel-sourced tables in warehouseOutbound (executionCardData,
contractData, customerProductType) and their data are left untouched.

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

uptime_kuma_utils.py

@@ -8,7 +8,7 @@ Uptime Kuma 心跳监控工具
   心跳的网络耗时 / 重试不会阻塞业务逻辑。
 - 单次心跳自带重试 (MAX_RETRIES), 容忍 Uptime Kuma 服务偶发的慢响应 / 超时 / 4xx。
 - send_heartbeat() / check_and_send_heartbeat() / send_stop_signal() 保留为
-  向后兼容的同步接口 (excel_sync_to_sql.py 等仍在使用)。
+  向后兼容的同步接口。
 """
 
 import time