feat: Finalizer-Agent – dritter Agent der nach Worker + Review einschreitet

Backend:
- SessionRecord + DB-Migration: finalizer_agent_id auf sessions
- ChatContext.FinalizerContext (Model, SystemPrompt, Tools, Workspace)
- ChatService: FinalizerContext aufbauen (analog zu ReviewContext)
- AnthropicService: RunFinalizer() nach Review-Phase (oder direkt nach Worker)
- RunReviewAgentChat() refactored zu generischem RunAgentChat()
  mit agentType-Parameter ('review' | 'finalizer') für SSE-Events und DB-Tags
- SSE-Events: finalizer_start, finalizer_text, finalizer_tool_use, finalizer_done

Frontend:
- Session-Page: Finalizer-Agent-Picker (im 'Weitere Einstellungen' Bereich)
- Chat: SSE-Events für Finalizer-Phase verarbeiten
- UI: addReviewBubble/finalizeReviewBubble unterstützen agentType 'finalizer'
  mit eigener Farbe (success-green) und Flag-Checkered-Icon
- renderMarkdown: [Finalizer: AgentName] Tags parsen
- History: Finalizer-Messages als .finalizer-bubble rendern
- Shared-Page: Finalizer-Messages korrekt erkennen und stylen
- Dashboard: Finalizer-Badge (flag-checkered) auf Session-Cards

Services/ChatService.cs

@@ -142,6 +142,41 @@ public class ChatService
             }
         }
 
+        // Finalizer-Agent vorbereiten (falls konfiguriert)
+        FinalizerContext finalizerContext = null;
+        if (!string.IsNullOrEmpty(session.FinalizerAgentId))
+        {
+            var finalizerAgent = await _db.GetAgent(session.FinalizerAgentId);
+            if (finalizerAgent != null)
+            {
+                var finalizerModelId = ResolveModelId(finalizerAgent.Model);
+                var finalizerMarked = await _db.GetMarkedMessages(sessionId);
+                var finalizerMemDir = _memoryService.GetAgentMemoryDir(finalizerAgent.AgentId);
+                var finalizerSkills = await _db.GetAgentSkills(finalizerAgent.AgentId);
+                var finalizerWorkspace = workspace;
+                var finalizerSystemText = SystemPromptBuilder.Build(
+                    finalizerAgent, session, finalizerMarked, _memoryService, finalizerMemDir,
+                    finalizerSkills, _skillService, userPersona: user?.GetEffectivePersona(),
+                    resolvedWorkspace: finalizerWorkspace);
+
+                var finalizerPrompt = new List<object>();
+                if (!string.IsNullOrWhiteSpace(finalizerSystemText))
+                    finalizerPrompt.Add(new { type = "text", text = finalizerSystemText });
+
+                finalizerContext = new FinalizerContext
+                {
+                    ModelId = finalizerModelId,
+                    SystemPrompt = finalizerPrompt,
+                    AgentId = finalizerAgent.AgentId,
+                    Workspace = finalizerWorkspace,
+                    UseMemory = finalizerAgent.UseMemory,
+                    WebSearchVersion = finalizerAgent.WebSearchVersion,
+                    ToolChoice = finalizerAgent.ToolChoice ?? "any",
+                    Agent = finalizerAgent
+                };
+            }
+        }
+
         return new ChatContext
         {
             ChainId = chainId,
@@ -157,7 +192,8 @@ public class ChatService
             MaxIterations = maxIterations,
             Agent = agent,
             Session = session,
-            Review = reviewContext
+            Review = reviewContext,
+            Finalizer = finalizerContext
         };
     }