Fix _meta field format for MCP spec compliance (#367)

## Summary

Updates the `_meta` field structure to be compliant with the MCP
specification by using proper `prefix/name` format for vendor
extensions.

### Changes

- **Registry metadata**: `io.modelcontextprotocol.registry` →
`io.modelcontextprotocol.registry/official`
- **Publisher metadata**: `publisher` →
`io.modelcontextprotocol.registry/publisher-provided`
- **Database migration**: Updates indexes to use new field paths
- **Updated all code**: Service layer, validators, tests, and seed data

### Format Before
```json
{
  "_meta": {
    "publisher": { "license": "MIT" },
    "io.modelcontextprotocol.registry": { "id": "...", "is_latest": true }
  }
}
```

### Format After
```json
{
  "_meta": {
    "io.modelcontextprotocol.registry/publisher-provided": { "license": "MIT" },
    "io.modelcontextprotocol.registry/official": { "id": "...", "is_latest": true }
  }
}
```

This resolves the naming collision with the planned top-level
`publisher` field and ensures compliance with MCP specification
requirements.

Fixes #356

pkg/api/v0/types.go

@@ -23,8 +23,8 @@ type ServerListResponse struct {
 
 // ServerMeta represents the structured metadata with known extension fields
 type ServerMeta struct {
-	Publisher                      map[string]interface{} `json:"publisher,omitempty"`
-	IOModelContextProtocolRegistry *RegistryExtensions    `json:"io.modelcontextprotocol.registry,omitempty"`
+	Official         *RegistryExtensions    `json:"io.modelcontextprotocol.registry/official,omitempty"`
+	PublisherProvided map[string]interface{} `json:"io.modelcontextprotocol.registry/publisher-provided,omitempty"`
 }
 
 // ServerJSON represents complete server information as defined in the MCP spec, with extension support