Fix access token collision and cross-file username collision

- auth.py: _create_access_token had no jti claim, so two refreshes
  within the same wall-clock second produced byte-identical access
  tokens (same failure mode already fixed for refresh tokens earlier)
- test_refresh_endpoint.py: usernames (jwttoken, refreshuser, etc.)
  collided with test_auth_endpoints.py's own test usernames once run
  in the same pytest session against a shared DB with no per-test
  isolation -- exactly how run_checkpoint_tests.py invokes pytest.
  Namespaced this file's usernames with a refep_ prefix.

Full backend suite (63 tests) passes clean end-to-end against a real
Postgres with real migrations applied.

backend/tests/test_refresh_endpoint.py

@@ -67,7 +67,7 @@ class TestRefresh:
 
     def test_refresh_success(self):
         """A valid refresh token returns a new access token."""
-        refresh_token = self._register_user("refreshuser", "secret123")
+        refresh_token = self._register_user("refep_refreshuser", "secret123")
         client = TestClient(app)
         resp = client.post(
             "/api/auth/refresh",
@@ -82,7 +82,7 @@ class TestRefresh:
 
     def test_refresh_returns_different_access_token(self):
         """Refreshing produces a new access token (different from any previous)."""
-        refresh_token = self._register_user("refreshtoken", "secret123")
+        refresh_token = self._register_user("refep_refreshtoken", "secret123")
         client = TestClient(app)
 
         # First refresh
@@ -140,7 +140,7 @@ class TestRefresh:
         client.post(
             "/api/auth/register",
             json={
-                "username": "loginrefuser",
+                "username": "refep_loginrefuser",
                 "email": "loginref@example.com",
                 "password": "secret123",
                 "date_of_birth": "2000-01-01",
@@ -149,7 +149,7 @@ class TestRefresh:
         # Login to get a refresh token
         login_resp = client.post(
             "/api/auth/login",
-            data={"username": "loginrefuser", "password": "secret123"},
+            data={"username": "refep_loginrefuser", "password": "secret123"},
         )
         assert login_resp.status_code == 200
         refresh_token = login_resp.json()["refresh_token"]
@@ -166,7 +166,7 @@ class TestRefresh:
         """The access token returned by /refresh is a valid JWT."""
         from jose import jwt
 
-        refresh_token = self._register_user("jwttoken", "secret123")
+        refresh_token = self._register_user("refep_jwttoken", "secret123")
         client = TestClient(app)
         resp = client.post(
             "/api/auth/refresh",
@@ -190,7 +190,7 @@ class TestRefresh:
 
     def test_refresh_expires_minutes_in_response(self):
         """The response includes access_token_expires_minutes."""
-        refresh_token = self._register_user("expiretest", "secret123")
+        refresh_token = self._register_user("refep_expiretest", "secret123")
         client = TestClient(app)
         resp = client.post(
             "/api/auth/refresh",