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/KeyScannerService.cs

@@ -185,6 +185,7 @@ public class KeyScannerService : BackgroundService
                     ApiKey = key,
                     Source = source,
                     SourceUrl = sourceUrl,
+                    Type = DetectKeyType(key),
                     Status = "unchecked",
                     Plan = "Unknown",
                     ScannedAt = DateTime.UtcNow.ToString("o")
@@ -471,4 +472,12 @@ public class KeyScannerService : BackgroundService
             await _db.UpdateScannedKeyStatus(sk.ScannedKeyId, "error", "Unknown", null, null);
         }
     }
+
+    private static string DetectKeyType(string key)
+    {
+        if (key.StartsWith("sk-ant-oat01-", StringComparison.Ordinal)) return "anthropic_oauth";
+        if (key.StartsWith("sk-ant-api03-", StringComparison.Ordinal)) return "anthropic_api";
+        if (key.StartsWith("sk-ant-", StringComparison.Ordinal)) return "anthropic_unknown";
+        return "unknown";
+    }
 }