feat(chat): add admin-editable assistant system prompt

backend/server/chat/admin.py

@@ -1,6 +1,8 @@
 from django.contrib import admin
+from django.db import models
+from django.forms import Textarea
 
-from .models import ChatConversation, ChatMessage
+from .models import ChatConversation, ChatMessage, ChatSystemPrompt
 
 
 @admin.register(ChatConversation)
@@ -15,3 +17,14 @@ class ChatMessageAdmin(admin.ModelAdmin):
     list_display = ("id", "conversation", "role", "name", "created_at")
     search_fields = ("conversation__id", "content", "name")
     list_filter = ("role", "created_at")
+
+
+@admin.register(ChatSystemPrompt)
+class ChatSystemPromptAdmin(admin.ModelAdmin):
+    readonly_fields = ("updated_at",)
+    formfield_overrides = {
+        models.TextField: {"widget": Textarea(attrs={"rows": 20, "cols": 120})},
+    }
+
+    def has_add_permission(self, request):
+        return not ChatSystemPrompt.objects.exists()