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

Services/DatabaseService.cs

@@ -330,6 +330,7 @@ public partial class DatabaseService
         TryAddColumn(conn, "scanned_keys", "deleted", "INTEGER NOT NULL DEFAULT 0");
         TryAddColumn(conn, "scanned_keys", "adopted_api_key_id", "INTEGER");
         TryAddColumn(conn, "scanned_keys", "source", "TEXT");
+        TryAddColumn(conn, "scanned_keys", "type", "TEXT NOT NULL DEFAULT 'anthropic_oauth'");
 
         // api_keys Metadaten-Spalten
         TryAddColumn(conn, "api_keys", "status", "TEXT");