Fix: 0 credits for farewell chat messages

Abschlussnachrichten (danke, ok, tschüss, passt, ...) kosten 0 statt 5
Credits — keine Leistung erbracht, keine Verrechnung.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/Http/Controllers/Api/AgentChatController.php

@@ -195,8 +195,15 @@ class AgentChatController extends Controller
                 $credits = $this->calculateCredits($usage, $aiConfig, $type);
             }
         } else {
-            // Chat: immer 5 Credits pauschal
-            $credits = 5;
+            // Chat: 5 Credits — außer bei Abschlussnachrichten (0 Credits)
+            $farewells = ['danke', 'tschüss', 'tschuss', 'bye', 'ciao',
+                          'ok', 'okay', 'alles klar', 'super', 'perfekt',
+                          'das war', 'nein', 'nix', 'nichts', 'passt'];
+            $msgLower  = mb_strtolower($request->message);
+            $isFarewell = collect($farewells)
+                ->contains(fn($w) => str_contains($msgLower, $w));
+
+            $credits = $isFarewell ? 0 : 5;
         }
 
         if ($shouldLog) {