Add comprehensive Podman secrets support to prevent credential exposure:
New types and methods (internal/container/podman.go):
- PodmanSecret struct for secret definitions
- CreateSecret() - Create Podman secrets from sensitive data
- DeleteSecret() - Clean up secrets after use
- BuildSecretArgs() - Generate podman run arguments for secrets
- SanitizeContainerEnv() - Extract sensitive env vars as secrets
- ContainerConfig.Secrets field for secret list
Enhanced container lifecycle:
- StartContainer() now creates secrets before starting container
- Secrets automatically mounted via --secret flag
- Cleanup on failure to prevent secret leakage
- Secrets logged as count only (not content)
Jupyter service integration (internal/jupyter/service_manager.go):
- prepareContainerConfig() uses SanitizeContainerEnv()
- JUPYTER_TOKEN and JUPYTER_PASSWORD now use secrets
- Maintains backward compatibility with env var mounting
Security benefits:
- Credentials no longer visible in 'podman inspect' output
- Secrets not exposed via /proc/*/environ inside container
- Automatic cleanup prevents secret accumulation
- Compatible with existing Jupyter authentication
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