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/importer/importer_test.go

@@ -33,7 +33,7 @@ func TestImportService_LocalFile(t *testing.T) {
 				Version: "1.0.0",
 			},
 			Meta: &apiv0.ServerMeta{
-				IOModelContextProtocolRegistry: &apiv0.RegistryExtensions{
+				Official: &apiv0.RegistryExtensions{
 					ID:          "test-id-1",
 					PublishedAt: time.Now(),
 					UpdatedAt:   time.Now(),
@@ -79,7 +79,7 @@ func TestImportService_HTTPFile(t *testing.T) {
 				Version: "2.0.0",
 			},
 			Meta: &apiv0.ServerMeta{
-				IOModelContextProtocolRegistry: &apiv0.RegistryExtensions{
+				Official: &apiv0.RegistryExtensions{
 					ID:          "test-id-2",
 					PublishedAt: time.Now(),
 					UpdatedAt:   time.Now(),
@@ -124,7 +124,7 @@ func TestImportService_RegistryAPI(t *testing.T) {
 				Version: "1.0.0",
 			},
 			Meta: &apiv0.ServerMeta{
-				IOModelContextProtocolRegistry: &apiv0.RegistryExtensions{
+				Official: &apiv0.RegistryExtensions{
 					ID:          "api-test-id-1",
 					PublishedAt: time.Now(),
 					UpdatedAt:   time.Now(),
@@ -144,7 +144,7 @@ func TestImportService_RegistryAPI(t *testing.T) {
 				Version: "2.0.0",
 			},
 			Meta: &apiv0.ServerMeta{
-				IOModelContextProtocolRegistry: &apiv0.RegistryExtensions{
+				Official: &apiv0.RegistryExtensions{
 					ID:          "api-test-id-2",
 					PublishedAt: time.Now(),
 					UpdatedAt:   time.Now(),