feat: multi-provider SMS, endpoint edit/auto-check, credential edit/headers

- SMS alert: multi-provider support (Verimor, Netgsm, Twilio, Vonage,
  Ileti Merkezi, Mutlucell) with provider-specific config and UI
- Endpoint auto-check: automatically run schema check when endpoint is created
- Endpoint editing: edit method, path, schedule via modal and PUT API
- Credential editing: edit existing credential values via modal
- Multiple custom credentials: add multiple key-value pairs at once with + button
- Path param substitution: resolve {paramName} from credentials in fetcher
- Custom HTTP headers: header:* prefixed credentials injected as request headers
- Updated tests for multi-provider SMS design (100 tests pass)

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

Author: halilibrahimd27