feat: Nachrichten als gelöscht markieren

- DB: deleted_on TEXT Spalte in messages (Migration)
- Backend: ToggleMessageDelete(), includeDeleted-Parameter in allen History-Queries
- Neuer Endpoint: PUT /api/messages/{id}/delete
- Frontend: Trash-Button zwischen Reply- und Mark-Button
- Gelöschte Nachrichten: durchgestrichen + roter Rand (oder versteckt je nach Einstellung)
- Einstellungen: Toggle-Switch 'Gelöschte Nachrichten anzeigen'
- KI-Payload: gelöschte Nachrichten werden nicht mitgeschickt

Endpoints/HistoryEndpoint.cs

@@ -21,6 +21,7 @@ public static class HistoryEndpoint
         app.MapGet("/api/archive/{archiveId}", HandleGetArchive);
         app.MapGet("/api/attachments/{messageId}", HandleGetAttachments);
         app.MapPut("/api/messages/{messageId}/mark", HandleToggleMark);
+        app.MapPut("/api/messages/{messageId}/delete", HandleToggleDelete);
         app.MapGet("/api/chains/{chainId}", HandleGetChain);
     }
 
@@ -44,23 +45,24 @@ public static class HistoryEndpoint
         const int messageLimit = 500;
         var beforeStr = ctx.Request.Query["beforeId"].FirstOrDefault();
         var beforeId = long.TryParse(beforeStr, out var b) ? b : 0;
+        var includeDeleted = ctx.Request.Query["includeDeleted"].FirstOrDefault() == "true";
 
-        var totalCount = await db.GetActiveMessageCount(sessionId);
+        var totalCount = await db.GetActiveMessageCount(sessionId, includeDeleted);
 
         List<MessageRecord> messages;
         if (beforeId > 0)
         {
-            messages = await db.GetActiveMessagesBeforeId(sessionId, beforeId, messageLimit);
+            messages = await db.GetActiveMessagesBeforeId(sessionId, beforeId, messageLimit, includeDeleted);
         }
         else
         {
-            messages = await db.GetLatestActiveMessages(sessionId, messageLimit);
+            messages = await db.GetLatestActiveMessages(sessionId, messageLimit, includeDeleted);
         }
 
         // Zähle wie viele Nachrichten es VOR der ältesten geladenen noch gibt
         var oldestLoadedId = messages.Count > 0 ? messages[0].Id : 0;
         var remainingOlder = oldestLoadedId > 0
-            ? await db.GetActiveMessageCountBeforeId(sessionId, oldestLoadedId)
+            ? await db.GetActiveMessageCountBeforeId(sessionId, oldestLoadedId, includeDeleted)
             : 0;
 
         // Nach Chain gruppieren (Reihenfolge beibehalten)
@@ -91,6 +93,7 @@ public static class HistoryEndpoint
         var sessionId = ctx.Request.Query["sessionId"].FirstOrDefault();
         var afterStr = ctx.Request.Query["afterId"].FirstOrDefault();
         var afterId = long.TryParse(afterStr, out var a) ? a : 0;
+        var includeDeleted = ctx.Request.Query["includeDeleted"].FirstOrDefault() == "true";
 
         if (string.IsNullOrEmpty(sessionId) || !await VerifySessionOwner(ctx, db, sessionId))
         {
@@ -98,7 +101,7 @@ public static class HistoryEndpoint
             return;
         }
 
-        var messages = await db.GetActiveMessagesAfterId(sessionId, afterId);
+        var messages = await db.GetActiveMessagesAfterId(sessionId, afterId, includeDeleted: includeDeleted);
         await ctx.Response.WriteAsJsonAsync(messages);
     }
 
@@ -174,6 +177,19 @@ public static class HistoryEndpoint
         await ctx.Response.WriteAsJsonAsync(new { messageId, isMarked });
     }
 
+    private static async Task HandleToggleDelete(HttpContext ctx, DatabaseService db)
+    {
+        var messageIdStr = ctx.Request.RouteValues["messageId"]?.ToString();
+        if (!long.TryParse(messageIdStr, out var messageId))
+        {
+            ctx.Response.StatusCode = 400;
+            return;
+        }
+
+        var deletedOn = await db.ToggleMessageDelete(messageId);
+        await ctx.Response.WriteAsJsonAsync(new { messageId, deletedOn });
+    }
+
     private static async Task HandleGetChain(HttpContext ctx, DatabaseService db)
     {
         var chainId = ctx.Request.RouteValues["chainId"]?.ToString();