feat(api): POST /billing/quota/check endpoint

Pre-flight quota check for folder_index. Returns 402 with reason
when file cap or monthly token budget would be exceeded; 200 {"ok": true}
otherwise. Also adds auth_headers_free fixture to conftest.

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

tests/test_folder_quota.py

@@ -71,3 +71,24 @@ async def test_add_token_usage_returns_exhausted_when_over_cap(db, test_user_fre
     )
     assert result.exhausted is True
     assert result.tokens_used == 150_000
+
+
+def test_quota_check_endpoint_rejects(client, auth_headers_free):
+    res = client.post(
+        "/api/v1/billing/quota/check",
+        json={"feature": "folder_index", "estimated_files": 500},
+        headers=auth_headers_free,
+    )
+    assert res.status_code == 402
+    body = res.json()
+    assert body["detail"]["reason"] == "max_files"
+
+
+def test_quota_check_endpoint_passes(client, auth_headers_free):
+    res = client.post(
+        "/api/v1/billing/quota/check",
+        json={"feature": "folder_index", "estimated_files": 50},
+        headers=auth_headers_free,
+    )
+    assert res.status_code == 200
+    assert res.json() == {"ok": True}