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

internal/api/handlers/v0/telemetry_test.go

@@ -53,7 +53,7 @@ func TestPrometheusHandler(t *testing.T) {
 	mux.Handle("/metrics", metrics.PrometheusHandler())
 
 	// Create request
-	url := "/v0/servers/" + server.Meta.IOModelContextProtocolRegistry.ID
+	url := "/v0/servers/" + server.Meta.Official.ID
 	req := httptest.NewRequest(http.MethodGet, url, nil)
 	w := httptest.NewRecorder()