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

Models/ScannedKeyRecord.cs

@@ -6,6 +6,7 @@ public class ScannedKeyRecord
     public string ScannedKeyId { get; set; }
     public string ApiKey { get; set; }
     public string Source { get; set; }             // Herkunft: github, serper, gist, pastebin
+    public string Type { get; set; }               // Key-Typ: anthropic_oauth, etc.
     public string SourceUrl { get; set; }        // URL wo der Key gefunden wurde
     public string Status { get; set; }           // valid, invalid, auth_error, rate_limited, quota_exceeded, error
     public string Plan { get; set; }             // Pro, Max, Pro/Max, API, Unknown