feat(core): dynamic settings service, IFinlyticLogger, log broadcaster, and persistent Yahoo auth

FinlyticCore/Dtos/Logging/LogMessageDto.cs

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+using System;
+using System.Text.Json.Serialization;
+
+namespace FinlyticCore.Dtos.Logging;
+
+public record LogMessageDto(
+    [property: JsonPropertyName("timestamp")] DateTime Timestamp,
+    [property: JsonPropertyName("serviceName")] string ServiceName,
+    [property: JsonPropertyName("channel")] string Channel,
+    [property: JsonPropertyName("level")] string Level,
+    [property: JsonPropertyName("message")] string Message,
+    [property: JsonPropertyName("exception")] string? Exception = null
+);

FinlyticCore/Dtos/Settings/DynamicSettingDto.cs

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+using System;
+
+namespace FinlyticCore.Dtos.Settings;
+
+/// <summary>
+/// Repräsentiert eine dynamische Einstellung für das Web-UI und MQTT-RPC.
+/// </summary>
+public record DynamicSettingDto(
+    string Key,
+    object? Value,
+    string Type,
+    string Description = "",
+    DateTime? UpdatedAt = null
+);