feat: HTTP/SOCKS Proxy-Support für alle HttpClient-Requests

- Program.cs: ProxyUrl aus Config lesen, WebProxy auf allen HttpClients via ConfigureHttpClientDefaults
- ServerSettingsEndpoint: ProxyUrl Setting im Frontend (Text-Feld)
- appsettings.json: ProxyUrl Default (leer = kein Proxy)
- Unterstützt HTTP und SOCKS5 Proxies (z.B. socks5://127.0.0.1:1080)
- Betrifft alle ausgehenden Requests: Anthropic, OpenAI, GitHub, Serper, Telegram, Krea

Endpoints/ServerSettingsEndpoint.cs

@@ -33,6 +33,7 @@ public static class ServerSettingsEndpoint
         new SettingDefinition("KreaApiKey", "string", "", "API-Key für Krea-Endpoints (Automation)", "password"),
         new SettingDefinition("GitHubToken", "string", "", "GitHub PAT für Key-Scanner", "password"),
         new SettingDefinition("SerperApiKey", "string", "", "Serper.dev API-Key für Google-Suche (Key-Scanner)", "password"),
+        new SettingDefinition("ProxyUrl", "string", "", "HTTP/SOCKS Proxy für alle Requests (z.B. socks5://127.0.0.1:1080)", "text"),
         new SettingDefinition("ScanIntervalHours", "int", 0, "Key-Scanner Intervall in Stunden (0 = nur manuell)", "number"),
     };