feat: add AI suggestion audit trail endpoints

Create/list/resolve endpoints for tracking AI-applied changes to flows.

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

backend/tests/test_ai_suggestions.py

@@ -0,0 +1,41 @@
+"""Tests for AI suggestion endpoints."""
+import pytest
+
+
+@pytest.mark.asyncio
+async def test_create_and_list_suggestions(client, auth_headers, test_tree):
+    """Can create and list suggestions for a tree."""
+    tree_id = test_tree["id"]
+
+    # Create suggestion
+    resp = await client.post(
+        "/api/v1/ai/suggestions",
+        json={
+            "tree_id": tree_id,
+            "action_type": "generate_branch",
+            "target_node_id": "some-node",
+            "changes_json": {"before": {}, "after": {"id": "new-node"}},
+        },
+        headers=auth_headers,
+    )
+    assert resp.status_code == 201
+    suggestion_id = resp.json()["id"]
+    assert resp.json()["status"] == "pending"
+
+    # List suggestions
+    resp = await client.get(
+        f"/api/v1/ai/suggestions/tree/{tree_id}",
+        headers=auth_headers,
+    )
+    assert resp.status_code == 200
+    assert len(resp.json()) >= 1
+
+    # Resolve suggestion
+    resp = await client.patch(
+        f"/api/v1/ai/suggestions/{suggestion_id}",
+        json={"status": "accepted"},
+        headers=auth_headers,
+    )
+    assert resp.status_code == 200
+    assert resp.json()["status"] == "accepted"
+    assert resp.json()["resolved_at"] is not None