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

backend/server/chat/migrations/0002_chatsystemprompt.py

@@ -0,0 +1,37 @@
+# Generated by Django 5.2.12 on 2026-03-10
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("chat", "0001_initial"),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name="ChatSystemPrompt",
+            fields=[
+                (
+                    "id",
+                    models.BigAutoField(
+                        auto_created=True,
+                        primary_key=True,
+                        serialize=False,
+                        verbose_name="ID",
+                    ),
+                ),
+                (
+                    "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)),
+            ],
+            options={
+                "verbose_name": "Chat System Prompt",
+                "verbose_name_plural": "Chat System Prompt",
+            },
+        ),
+    ]