Add Conversation + Turn entities and migration

backend/Conversations/Conversation.cs

@@ -0,0 +1,19 @@
+namespace backend.Conversations;
+
+public enum EndReason
+{
+    Unknown = 0,
+    Tool = 1,
+    Idle = 2,
+    Error = 3,
+}
+
+public class Conversation
+{
+    public Guid Id { get; set; } = Guid.NewGuid();
+    public Guid DeviceId { get; set; }
+    public DateTimeOffset StartedAt { get; set; }
+    public DateTimeOffset? EndedAt { get; set; }
+    public EndReason EndReason { get; set; } = EndReason.Unknown;
+    public List<Turn> Turns { get; set; } = new();
+}

backend/Conversations/Turn.cs

@@ -0,0 +1,21 @@
+namespace backend.Conversations;
+
+public enum TurnRole
+{
+    User = 0,
+    Assistant = 1,
+    Tool = 2,
+}
+
+public class Turn
+{
+    public Guid Id { get; set; } = Guid.NewGuid();
+    public Guid ConversationId { get; set; }
+    public int Index { get; set; }
+    public TurnRole Role { get; set; }
+    public string? Text { get; set; }
+    public string? ToolName { get; set; }
+    public string? ToolArgsJson { get; set; }
+    public string? ToolResultJson { get; set; }
+    public DateTimeOffset CreatedAt { get; set; }
+}