feat: Nachrichten als wichtig markieren (Stern-Button)

- MessageRecord: neues Property IsMarked
- DatabaseService: Migration is_marked, ToggleMessageMark(), GetMarkedMessages()
- HistoryEndpoint: PUT /api/messages/{id}/mark (Toggle)
- SystemPromptBuilder: markierte Nachrichten als Referenz im System-Prompt
- ChatEndpoint + SessionEndpoint: markierte Nachrichten laden und an Builder übergeben
- Frontend: Stern-Button (☆/★) auf jeder Nachricht, Toggle per API
- CSS: .marked-Klasse mit gelbem Seitenrand, Button-Styling

Endpoints/ChatEndpoint.cs

@@ -77,7 +77,8 @@ public static class ChatEndpoint
         res.Headers["Cache-Control"] = "no-cache";
         res.Headers["Connection"] = "keep-alive";
 
-        var systemPromptText = SystemPromptBuilder.Build(agent, session);
+        var markedMessages = await db.GetMarkedMessages(sessionId);
+        var systemPromptText = SystemPromptBuilder.Build(agent, session, markedMessages);
 
         var chainId = Guid.NewGuid().ToString("N");
 

Endpoints/HistoryEndpoint.cs

@@ -19,6 +19,7 @@ public static class HistoryEndpoint
         app.MapGet("/api/archives", HandleListArchives);
         app.MapGet("/api/archive/{archiveId}", HandleGetArchive);
         app.MapGet("/api/attachments/{messageId}", HandleGetAttachments);
+        app.MapPut("/api/messages/{messageId}/mark", HandleToggleMark);
         app.MapGet("/api/chains/{chainId}", HandleGetChain);
     }
 
@@ -119,6 +120,19 @@ public static class HistoryEndpoint
         await ctx.Response.WriteAsJsonAsync(result);
     }
 
+    private static async Task HandleToggleMark(HttpContext ctx, DatabaseService db)
+    {
+        var messageIdStr = ctx.Request.RouteValues["messageId"]?.ToString();
+        if (!long.TryParse(messageIdStr, out var messageId))
+        {
+            ctx.Response.StatusCode = 400;
+            return;
+        }
+
+        var isMarked = await db.ToggleMessageMark(messageId);
+        await ctx.Response.WriteAsJsonAsync(new { messageId, isMarked });
+    }
+
     private static async Task HandleGetChain(HttpContext ctx, DatabaseService db)
     {
         var chainId = ctx.Request.RouteValues["chainId"]?.ToString();

Endpoints/SessionEndpoint.cs

@@ -122,7 +122,8 @@ public static class SessionEndpoint
             return;
         }
 
-        var systemPrompt = SystemPromptBuilder.Build(agent, session);
+        var markedMessages = await db.GetMarkedMessages(sessionId);
+        var systemPrompt = SystemPromptBuilder.Build(agent, session, markedMessages);
 
         ctx.Response.ContentType = "text/plain; charset=utf-8";
         await ctx.Response.WriteAsync(systemPrompt);