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

wwwroot/index.html

@@ -580,8 +580,8 @@
           </div>
         </div>
         <div id="scannedKeysList" class="admin-user-list"></div>
-        <div id="scannedKeysActions" style="padding:.5rem 0;display:none">
-          <button id="scannedDeleteAllBtn" class="icon-btn" title="Alle gescannten Keys löschen" style="font-size:.85em;color:var(--danger)"><i class="fa fa-trash"></i> Alle löschen</button>
+        <div class="form-actions">
+          <button hidden>&nbsp;</button>
         </div>
       </div>
     </div>