test(auth): add test_existing_endpoints_require_auth + per-test AUTH_DISABLED flips

- New test_existing_endpoints_require_auth.py: spot-check that existing
  /api/* endpoints now require auth (gated via Depends(matrix_gate)) when
  AUTH_DISABLED is False. Health remains public.

- conftest.py: flip AUTH_DISABLED=True for the suite so the legacy
  pre-auth tests keep passing without login. Auth tests flip it back
  off via their own autouse fixture (now patched to use monkeypatch
  for cleanup).

Verified: 53 auth tests pass; 222 pre-existing non-auth failures are
unchanged.

backend/tests/test_existing_endpoints_require_auth.py

@@ -0,0 +1,70 @@
+"""Spot-check that existing endpoints now require auth (when AUTH_DISABLED is not set).
+
+The conftest in ``tests/conftest.py`` flips ``AUTH_DISABLED=True`` for
+every test module that does NOT start with ``test_auth`` — this module
+deliberately is NOT named ``test_auth_*`` so it inherits the default
+disabled posture... wait, that's the opposite of what we want.
+
+This module is named ``test_existing_endpoints_require_auth`` so the
+conftest will treat it as a legacy test and set ``AUTH_DISABLED=True``,
+bypassing the auth check entirely. To actually verify the gate, this
+test's ``client`` fixture flips ``AUTH_DISABLED`` to ``False``
+*just for this test*, then restores it afterwards. This way the
+rest of the suite still sees the disabled posture and existing
+tests keep passing.
+"""
+
+from __future__ import annotations
+
+import pytest
+from fastapi.testclient import TestClient
+from sqlalchemy import delete
+
+from cyclone.api import app
+from cyclone.auth import deps as _auth_deps
+from cyclone.auth.deps import AUTH_DISABLED
+from cyclone.db import Session as DbSession
+from cyclone.db import SessionLocal, User
+
+
+@pytest.fixture(autouse=True)
+def _clear():
+    with SessionLocal()() as db:
+        db.execute(delete(DbSession))
+        db.execute(delete(User))
+        db.commit()
+    yield
+    with SessionLocal()() as db:
+        db.execute(delete(DbSession))
+        db.execute(delete(User))
+        db.commit()
+
+
+@pytest.fixture
+def client():
+    # Force AUTH_DISABLED=False for these tests so the dep actually checks the cookie.
+    original = AUTH_DISABLED
+    _auth_deps.AUTH_DISABLED = False
+    try:
+        yield TestClient(app)
+    finally:
+        _auth_deps.AUTH_DISABLED = original
+
+
+@pytest.mark.parametrize("method,path", [
+    ("GET", "/api/claims"),
+    ("GET", "/api/remittances"),
+    ("GET", "/api/providers"),
+    ("GET", "/api/batches"),
+    ("GET", "/api/payers/p1/summary"),
+    ("GET", "/api/activity"),
+])
+def test_existing_get_endpoints_require_auth(client, method, path):
+    resp = client.request(method, path)
+    assert resp.status_code == 401, f"{method} {path} returned {resp.status_code}"
+
+
+def test_health_is_public(client):
+    """``/api/health`` is the public healthcheck and must remain reachable."""
+    resp = client.get("/api/health")
+    assert resp.status_code != 401, f"/api/health returned {resp.status_code}"