refactor: MaxHistory + ReminderInterval in appsettings auslagern

- MaxHistory (Default: 120): Max History-Messages pro API-Request
- ReminderInterval (Default: 30): System-Prompt Reminder alle N Messages (0 = deaktiviert)
- Beide Werte über Server-Settings-UI konfigurierbar
- Hardcoded Werte in ChatService durch konfigurierbare Felder ersetzt

Endpoints/ServerSettingsEndpoint.cs

@@ -25,6 +25,8 @@ public static class ServerSettingsEndpoint
         new SettingDefinition("Port", "int", 19000, "Server-Port (erfordert Neustart)", "number"),
         new SettingDefinition("AnthropicToken", "string", "", "Anthropic API Key", "password"),
         new SettingDefinition("MaxIterations", "int", 100, "Max Tool-Loop Iterationen pro Chat", "number"),
+        new SettingDefinition("MaxHistory", "int", 120, "Max History-Messages pro Request", "number"),
+        new SettingDefinition("ReminderInterval", "int", 30, "System-Prompt Reminder alle N Messages (0 = deaktiviert)", "number"),
         new SettingDefinition("PayloadLogging", "bool", false, "API-Payloads loggen (Debug)", "toggle"),
         new SettingDefinition("PayloadLoggingGzip", "bool", false, "Payload-Logs als .gz komprimieren", "toggle"),
         new SettingDefinition("KreaToken", "string", "", "Krea API Token", "password"),