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/conftest.py

@@ -162,6 +162,12 @@ async def test_user_power(db_session: AsyncSession):
     return result.scalar_one()
 
 
+@pytest.fixture
+def auth_headers_free() -> dict[str, str]:
+    """Authorization header for the seeded free-tier user."""
+    return auth_header("free")
+
+
 # ── CLI options ───────────────────────────────────────────────────────
 
 def pytest_addoption(parser):