feat: auto-detect flow env vars in deployment connection form

- Add POST /deployments/variables/detections backend endpoint that scans
  flow version data for credential fields (load_from_db=True globals and
  password=True fallbacks) and returns detected variable names
- Derive meaningful env var names from the model field's category when no
  global variable is linked (e.g. OPENAI_API_KEY from category "OpenAI")
- Add DetectEnvVarsRequest/DetectedEnvVar/DetectEnvVarsResponse schemas
- Add usePostDetectDeploymentEnvVars frontend mutation hook
- Pre-populate Create Connection env var rows with detected keys/values
  when attaching a flow version; global variable selections render as tags
  via InputComponent with global variable picker support