refactor: Chat-Payload vereinfacht + Inline-Bilder statt Platzhalter

Backend:
- ChatEndpoint empfängt nur noch content + sessionId (nicht mehr die gesamte History)
- History wird komplett aus DB aufgebaut via BuildHistoryFromDb
- Gelöschte Nachrichten (deleted_on) werden aus History gefiltert
- Leere Nachrichten werden übersprungen (kein API 400 mehr)
- BuildHistoryFromDb optimiert: nur Text für ältere Chains, Tool-Calls nur für aktuelle Chain
- GetChainMessages: includeDeleted-Parameter (Default: false)
- Archiv-Export nutzt includeDeleted: true

Frontend:
- messages[]-Array komplett entfernt (kein Client-seitiger History-State mehr)
- apiChat() sendet nur content + sessionId
- Inline-Bilder: [Bild: name] wird durch <img> ersetzt (live mit DataURL, History per API-Nachladen)
- _autoLoadImageAttachments() für History/Archiv/ältere Nachrichten
- Klick auf Inline-Bild öffnet Vollbild-Modal

Endpoints/ChatEndpoint.cs

@@ -23,10 +23,10 @@ public static class ChatEndpoint
     private static async Task HandleChat(HttpRequest req, HttpResponse res, IConfiguration config, ChatService chatService, DatabaseService db, StreamingTracker streamingTracker, AnthropicService anthropic, CancellationToken cancellationToken)
     {
         var body = await JsonSerializer.DeserializeAsync<ChatRequest>(req.Body);
-        if (body == null || body.Messages == null || body.Messages.Count == 0)
+        if (body == null || body.Content.ValueKind == JsonValueKind.Undefined)
         {
             res.StatusCode = 400;
-            await res.WriteAsJsonAsync(new { error = "messages required" });
+            await res.WriteAsJsonAsync(new { error = "content required" });
             return;
         }
 
@@ -49,58 +49,35 @@ public static class ChatEndpoint
             return;
         }
 
+        // Neue User-Message aus Request-Content aufbereiten und speichern
+        var userMessage = new ChatMessage { Role = "user", Content = body.Content };
+        var textContent = userMessage.GetTextContent();
+        var attachmentBlocks = userMessage.GetAttachmentBlocks();
+
+        var attachmentRecords = attachmentBlocks.Select(b => new AttachmentRecord
+        {
+            Type = b.Type,
+            MediaType = b.MediaType,
+            FileName = b.FileName,
+            Data = b.Data
+        }).ToList();
+
+        await db.SaveMessageWithAttachments(new MessageRecord
+        {
+            ChainId = ctx.ChainId,
+            Iteration = 0,
+            Role = "user",
+            Type = "text",
+            Content = textContent
+        }, attachmentRecords);
+
+        // History komplett aus DB aufbauen (inkl. gerade gespeicherter Message)
+        var messages = await chatService.BuildHistoryFromDb(sessionId);
+
         res.Headers["Content-Type"] = "text/event-stream";
         res.Headers["Cache-Control"] = "no-cache";
         res.Headers["Connection"] = "keep-alive";
 
-        // Messages aus dem Request aufbereiten
-        var messages = new List<object>();
-        foreach (var msg in body.Messages)
-        {
-            JsonElement content = msg.Content;
-            if (msg.Content.ValueKind == JsonValueKind.Array)
-            {
-                var node = System.Text.Json.Nodes.JsonNode.Parse(msg.Content.GetRawText());
-                foreach (var block in node.AsArray())
-                {
-                    if (block is System.Text.Json.Nodes.JsonObject obj)
-                        obj.Remove("_file_name");
-                }
-                content = JsonSerializer.Deserialize<JsonElement>(node.ToJsonString());
-            }
-
-            messages.Add(new Dictionary<string, JsonElement>
-            {
-                ["role"] = JsonSerializer.Deserialize<JsonElement>($"\"{msg.Role}\""),
-                ["content"] = content
-            });
-        }
-
-        // Nur die letzte User-Message speichern (= die neue)
-        var lastUserMsg = body.Messages.LastOrDefault(m => m.Role == "user");
-        if (lastUserMsg != null)
-        {
-            var textContent = lastUserMsg.GetTextContent();
-            var attachmentBlocks = lastUserMsg.GetAttachmentBlocks();
-
-            var attachmentRecords = attachmentBlocks.Select(b => new AttachmentRecord
-            {
-                Type = b.Type,
-                MediaType = b.MediaType,
-                FileName = b.FileName,
-                Data = b.Data
-            }).ToList();
-
-            await db.SaveMessageWithAttachments(new MessageRecord
-            {
-                ChainId = ctx.ChainId,
-                Iteration = 0,
-                Role = "user",
-                Type = "text",
-                Content = textContent
-            }, attachmentRecords);
-        }
-
         streamingTracker.Start(sessionId, ctx.ChainId);
         try
         {

Endpoints/HistoryEndpoint.cs

@@ -222,7 +222,7 @@ public static class HistoryEndpoint
         var result = new List<object>();
         foreach (var chain in chains)
         {
-            var messages = await db.GetChainMessages(chain.ChainId);
+            var messages = await db.GetChainMessages(chain.ChainId, includeDeleted: true);
             result.Add(new { chain, messages });
         }