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>

.env.example

@@ -12,4 +12,3 @@ NTFY_TOKEN=
 
 # Uptime Kuma
 UPTIME_KUMA_PUSH_URL=
-EXCEL_SYNC_UPTIME_KUMA_PUSH_URL=