- Unified configuration format: All services now use the same image
configuration structure for consistency.

- Private registry support: Added imagePullSecrets to enable pulling
images from private registries.

- Per-service flexibility: Each service can override image-related
parameters independently.

### What problem does this PR solve?

_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._

### Type of change

- [ ] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

### What problem does this PR solve?

### Type of change

- [X] New Feature (non-breaking change which adds functionality)

### What problem does this PR solve?

Adds a new Kubernetes Service resource to the Helm chart which
specifically targets the RAGFlow API. This feature useful for cases
where you want to expose the RAGFlow HTTP API separately from the web
interface, for example if RAGFlow is running behind an authenticating
proxy it allows a route to bypass the proxy (e.g. by defining a separate
ingress resource which forwards to the separate API-only k8s service
added here) to provide RAGFlow API access. This is still secure since
API access is already authenticated by API keys inside RAGFlow itself.

### Type of change

- [X] New Feature (non-breaking change which adds functionality)

### What problem does this PR solve?

Add's a Helm chart for deploying RAGFlow on Kubernetes. 

Closes #864. 

### Type of change

- [X] New Feature (non-breaking change which adds functionality)