perf(locust): skip change-password in on_start when not required

Previously every user did login → change-pass → re-login in on_start
regardless of whether the server actually required a password change.
With bcrypt at ~250ms/call that's 3 bcrypt-bound requests per user.
At 2500 users the on_start queue was ~10k bcrypt ops — users never
escaped warmup, so @task endpoints never fired.

Login already returns must_change_password; only run the change-pass
+ re-login dance when the server says we have to. Cuts on_start from
3 requests to 1 for every user after the first DB initialization.

tests/stress/locustfile.py

@@ -24,7 +24,9 @@ class DecnetUser(HttpUser):
     wait_time = between(0.01, 0.05)  # near-zero think time — max pressure
 
     def _login_with_retry(self):
-        """Login with exponential backoff — handles connection storms."""
+        """Login with exponential backoff — handles connection storms.
+
+        Returns (access_token, must_change_password)."""
         for attempt in range(_MAX_LOGIN_RETRIES):
             resp = self.client.post(
                 "/api/v1/auth/login",
@@ -32,7 +34,8 @@ class DecnetUser(HttpUser):
                 name="/api/v1/auth/login [on_start]",
             )
             if resp.status_code == 200:
-                return resp.json()["access_token"]
+                body = resp.json()
+                return body["access_token"], bool(body.get("must_change_password", False))
             # Status 0 = connection refused, retry with backoff
             if resp.status_code == 0 or resp.status_code >= 500:
                 time.sleep(_LOGIN_BACKOFF_BASE * (2 ** attempt))
@@ -41,16 +44,20 @@ class DecnetUser(HttpUser):
         raise RuntimeError(f"Login failed after {_MAX_LOGIN_RETRIES} retries (last status: {resp.status_code})")
 
     def on_start(self):
-        token = self._login_with_retry()
+        token, must_change = self._login_with_retry()
 
-        # Clear must_change_password
-        self.client.post(
-            "/api/v1/auth/change-password",
-            json={"old_password": ADMIN_PASS, "new_password": ADMIN_PASS},
-            headers={"Authorization": f"Bearer {token}"},
-        )
-        # Re-login for a clean token
-        self.token = self._login_with_retry()
+        # Only pay the change-password + re-login cost on the very first run
+        # against a fresh DB. Every run after that, must_change_password is
+        # already False — skip it or the login path becomes a bcrypt storm.
+        if must_change:
+            self.client.post(
+                "/api/v1/auth/change-password",
+                json={"old_password": ADMIN_PASS, "new_password": ADMIN_PASS},
+                headers={"Authorization": f"Bearer {token}"},
+            )
+            token, _ = self._login_with_retry()
+
+        self.token = token
         self.client.headers.update({"Authorization": f"Bearer {self.token}"})
 
     # --- Read-hot paths (high weight) ---