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/app/routers/auth.py

@@ -66,8 +66,15 @@ class UserResponse(BaseModel):
 # --- Helpers ---
 
 def _create_access_token(data: dict) -> str:
-    """Create a signed access token using RS256."""
+    """Create a signed access token using RS256.
+
+    Includes a unique ``jti`` claim so that concurrent/same-second calls
+    (e.g. back-to-back /refresh requests) still produce distinct tokens —
+    without it, two tokens minted for the same user within the same
+    second are byte-identical since ``sub``/``exp`` are the only claims.
+    """
     to_encode = data.copy()
+    to_encode["jti"] = str(uuid.uuid4())
     expire = datetime.now(timezone.utc) + timedelta(
         minutes=settings.ACCESS_TOKEN_EXPIRE_MINUTES
     )