feat: API-Keys-Verwaltung in DB statt appsettings

- Neue Tabelle api_keys (Provider, Name, ApiKey, IsActive)
- Admin-CRUD: GET/POST/PUT/DELETE /api/admin/api-keys
- Aktivieren/Deaktivieren pro Provider (nur einer aktiv)
- Live-Token-Update im AnthropicService ohne Neustart
- Migration: AnthropicToken aus appsettings.Production.json → DB
- AnthropicToken aus ServerSettings-Definitionen + appsettings entfernt
- Frontend: API-Keys-Seite mit Erstellen/Bearbeiten/Löschen/Toggle
- Dashboard: API-Keys-Link für Admins

Endpoints/ServerSettingsEndpoint.cs

@@ -23,7 +23,6 @@ public static class ServerSettingsEndpoint
     private static readonly List<SettingDefinition> Definitions = new List<SettingDefinition>
     {
         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"),