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

backend/server/chat/models.py

@@ -45,3 +45,37 @@ class ChatMessage(models.Model):
 
     class Meta:
         ordering = ["created_at"]
+
+
+class ChatSystemPrompt(models.Model):
+    """Admin-editable system prompt for the travel assistant.
+
+    Only one instance should exist (singleton pattern via Django admin).
+    The prompt text replaces the hardcoded base prompt in get_system_prompt().
+    Dynamic user/party preference injection is appended automatically.
+    """
+
+    prompt_text = models.TextField(
+        help_text="Base system prompt for the travel assistant. "
+        "User and party travel preferences are appended automatically."
+    )
+    updated_at = models.DateTimeField(auto_now=True)
+
+    class Meta:
+        verbose_name = "Chat System Prompt"
+        verbose_name_plural = "Chat System Prompt"
+
+    def __str__(self):
+        return f"System Prompt (updated {self.updated_at})"
+
+    def save(self, *args, **kwargs):
+        self.pk = 1
+        super().save(*args, **kwargs)
+
+    @classmethod
+    def load(cls):
+        """Return the singleton instance or None if not configured."""
+        try:
+            return cls.objects.get(pk=1)
+        except cls.DoesNotExist:
+            return None