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/edit_test.go

@@ -42,9 +42,9 @@ func TestEditServerEndpoint(t *testing.T) {
 	assert.NoError(t, err)
 	assert.NotNil(t, published)
 	assert.NotNil(t, published.Meta)
-	assert.NotNil(t, published.Meta.IOModelContextProtocolRegistry)
+	assert.NotNil(t, published.Meta.Official)
 
-	testServerID := published.Meta.IOModelContextProtocolRegistry.ID
+	testServerID := published.Meta.Official.ID
 
 	// Publish a second server for permission testing
 	otherServer := apiv0.ServerJSON{
@@ -64,9 +64,9 @@ func TestEditServerEndpoint(t *testing.T) {
 	assert.NoError(t, err)
 	assert.NotNil(t, otherPublished)
 	assert.NotNil(t, otherPublished.Meta)
-	assert.NotNil(t, otherPublished.Meta.IOModelContextProtocolRegistry)
+	assert.NotNil(t, otherPublished.Meta.Official)
 
-	otherServerID := otherPublished.Meta.IOModelContextProtocolRegistry.ID
+	otherServerID := otherPublished.Meta.Official.ID
 
 	// Publish a deleted server for undelete testing
 	deletedServer := apiv0.ServerJSON{
@@ -86,9 +86,9 @@ func TestEditServerEndpoint(t *testing.T) {
 	assert.NoError(t, err)
 	assert.NotNil(t, deletedPublished)
 	assert.NotNil(t, deletedPublished.Meta)
-	assert.NotNil(t, deletedPublished.Meta.IOModelContextProtocolRegistry)
+	assert.NotNil(t, deletedPublished.Meta.Official)
 
-	deletedServerID := deletedPublished.Meta.IOModelContextProtocolRegistry.ID
+	deletedServerID := deletedPublished.Meta.Official.ID
 
 	testCases := []struct {
 		name           string

internal/api/handlers/v0/servers_test.go

@@ -175,8 +175,8 @@ func TestServersListEndpoint(t *testing.T) {
 						assert.NotEmpty(t, server.Name)
 						assert.NotEmpty(t, server.Description)
 						assert.NotNil(t, server.Meta)
-						assert.NotNil(t, server.Meta.IOModelContextProtocolRegistry)
-						assert.NotEmpty(t, server.Meta.IOModelContextProtocolRegistry.ID)
+						assert.NotNil(t, server.Meta.Official)
+						assert.NotEmpty(t, server.Meta.Official.ID)
 					}
 				}
 
@@ -223,7 +223,7 @@ func TestServersDetailEndpoint(t *testing.T) {
 	}{
 		{
 			name:           "successful get server detail",
-			serverID:       testServer.Meta.IOModelContextProtocolRegistry.ID,
+			serverID:       testServer.Meta.Official.ID,
 			expectedStatus: http.StatusOK,
 		},
 		{
@@ -305,7 +305,7 @@ func TestServersEndpointsIntegration(t *testing.T) {
 	assert.NoError(t, err)
 	assert.NotNil(t, published)
 
-	serverID := published.Meta.IOModelContextProtocolRegistry.ID
+	serverID := published.Meta.Official.ID
 	servers := []apiv0.ServerJSON{*published}
 	serverDetail := published
 

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()