Centralize credentials and make migration idempotent

- Move all DB credentials to config.asp with DSN/ODBC toggle
- Add Zabbix API URL and token to centralized config
- Update sql.asp, api.asp, apiusb.asp, zabbix.asp to use config
- Add GetConnectionString() and GetEmployeeConnectionString() functions
- Make migration SQL idempotent (safe to run multiple times)
- Add duplicate index cleanup (appname_2) to migration
- Document employee DB access limitation in CLAUDE.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -24,6 +24,7 @@ ShopDB is a Classic ASP/VBScript web application for managing manufacturing shop
 - Production IIS logs
 - Production MySQL database
 - Zabbix server (10.48.130.113)
+- Employee database (wjf_employees) - used by displayprofile.asp (can update code, cannot test)
 
 For production tasks, user must:
 - Relay production data/logs to Claude