Generate Manifest

Generate a manifest from natural language description.

Uses AI to create a valid YAML manifest from a natural language description
of desired resources. The generated manifest can be reviewed and applied.

**Example:**
```bash
curl -X POST /v1/manifest/generate \
  -H "Authorization: Bearer $API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "description": "I need a bucket of images that feeds into a multimodal extractor and a retriever",
    "manifest_name": "image-search-setup"
  }'
```

**Response:**
```json
{
  "manifest": "version: '1.0'

metadata: name: image-search-setup ...", "format": "yaml", "manifest_name": "image-search-setup", "description": "I need a bucket of images..." } ```

**Next Steps:**
1. Review the generated manifest
2. Apply it using POST /v1/manifest/apply