Telegram login hardening: 2FA lockout, DNS override, JWT warning, timeouts

[denis-kudelin]

Summary

• Login: on Telegram reminder failure return 503 with code=tg_unavailable when 2FA is enabled; otherwise set jwt.telegram_warning=true. Add warnings via logrus. (controller/account.go, model/vo/jwt.go)
• Telegram service: validate configs with detailed logs; use HTTP client with timeouts; optional DNS override via TELEGRAM_DNS_SERVERS; add time-bounded GetMe/SendWithMessage; return wrapped errors; expose function vars for tests. (service/telegram.go)
• Config: add TELEGRAM_DNS_SERVERS, TELEGRAM_2FA_ENABLE. (model/constant/config.go)
• DAO: guard nil sqlite handle in GetConfig. (dao/config.go)
• Tests: add TestLoginTelegramFailure to assert telegram_warning. (controller/login_test.go)
• Build: bump to Go 1.21 and update x/crypto, x/net, x/sys. (go.mod/go.sum)
• Docs: README notes on internal DNS behavior.