Add ConversationLog (start/append/end with monotonic per-convo index)

backend/Conversations/ConversationLog.cs

@@ -0,0 +1,63 @@
+using backend.Data;
+using Microsoft.EntityFrameworkCore;
+
+namespace backend.Conversations;
+
+public class ConversationLog(AppDbContext db)
+{
+    public async Task<Conversation> StartAsync(Guid deviceId, CancellationToken ct)
+    {
+        var c = new Conversation
+        {
+            DeviceId = deviceId,
+            StartedAt = DateTimeOffset.UtcNow,
+        };
+        db.Conversations.Add(c);
+        await db.SaveChangesAsync(ct);
+        return c;
+    }
+
+    public Task AppendUserAsync(Guid conversationId, string text, CancellationToken ct)
+        => AppendAsync(conversationId, TurnRole.User, text: text, ct: ct);
+
+    public Task AppendAssistantAsync(Guid conversationId, string text, CancellationToken ct)
+        => AppendAsync(conversationId, TurnRole.Assistant, text: text, ct: ct);
+
+    public Task AppendToolAsync(
+        Guid conversationId, string toolName, string argsJson, string resultJson, CancellationToken ct)
+        => AppendAsync(conversationId, TurnRole.Tool,
+            toolName: toolName, argsJson: argsJson, resultJson: resultJson, ct: ct);
+
+    public async Task EndAsync(Guid conversationId, EndReason reason, CancellationToken ct)
+    {
+        var row = await db.Conversations.FirstAsync(c => c.Id == conversationId, ct);
+        row.EndedAt = DateTimeOffset.UtcNow;
+        row.EndReason = reason;
+        await db.SaveChangesAsync(ct);
+    }
+
+    private async Task AppendAsync(
+        Guid conversationId, TurnRole role,
+        string? text = null, string? toolName = null,
+        string? argsJson = null, string? resultJson = null,
+        CancellationToken ct = default)
+    {
+        var next = await db.Turns
+            .Where(t => t.ConversationId == conversationId)
+            .Select(t => (int?)t.Index)
+            .MaxAsync(ct) ?? -1;
+
+        db.Turns.Add(new Turn
+        {
+            ConversationId = conversationId,
+            Index = next + 1,
+            Role = role,
+            Text = text,
+            ToolName = toolName,
+            ToolArgsJson = argsJson,
+            ToolResultJson = resultJson,
+            CreatedAt = DateTimeOffset.UtcNow,
+        });
+        await db.SaveChangesAsync(ct);
+    }
+}

backend/Program.cs

@@ -33,6 +33,7 @@ builder.Services.AddAuthorization();
 builder.Services.AddScoped<backend.Devices.DeviceTokenService>();
 builder.Services.AddScoped<backend.Pairing.PairingService>();
 builder.Services.AddSingleton<backend.Install.InstallScriptBuilder>();
+builder.Services.AddScoped<backend.Conversations.ConversationLog>();
 
 var app = builder.Build();