Add github_code_search tool + scanned key type field

- New tool: github_code_search (GitHub Code Search API with text-match fragments)
- Tool available when GitHubToken is configured in server settings
- Registered in AnthropicService + ChatService tool pipelines
- ToolService: SetHttpClientFactory() pattern for lazy DI resolution
- ScannedKeyRecord: type field (anthropic_oauth, anthropic_api, anthropic_unknown, unknown)
- DetectKeyType() based on key prefix
- DB migration: type column with default 'anthropic_oauth'
- Frontend: type label in scanned keys list

Endpoints/ApiKeysEndpoint.cs

@@ -263,6 +263,7 @@ public static class ApiKeysEndpoint
         {
             k.ScannedKeyId,
             k.Source,
+            k.Type,
             k.SourceUrl,
             k.Status,
             k.Plan,