Signed payment links, hashed refresh tokens, login-only leaderboard, load test tools

- /api/payment/orders/{id}/pay requires the short-lived signature that paymentUrl carries;
  without it (or with another order's) it is the same 404 as an unknown order
- Session.refresh_token stores the SHA-256 of the token; migration accounts.0007 hashes the
  existing rows so tokens clients already hold keep working
- /api/leaderboard requires a token (guests have one); names of children were scrapable
- RAG mock only for `manage.py test`, not any command with an argument named "test"
- tools/loadtest: user factory, load generator, fake GapGPT+Qdrant, with method notes
- security-before-production.md lists only open items; closed ones become a guards table
  with the tests that hold them. Comments describe current behaviour, not history

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

progress/tests_live_gamification.py

@@ -91,7 +91,7 @@ class ActivityFeedsStreakAndLeaderboardTests(TestCase):
         )
 
         for period in ('weekly', 'all_time'):
-            entries = self.client.get(f'/api/leaderboard?period={period}').json()['entries']
+            entries = self.client.get(f'/api/leaderboard?period={period}', **self.auth).json()['entries']
             self.assertEqual(entries[0]['userId'], str(self.user.id), period)
             self.assertEqual(entries[0]['name'], 'سارا')
             self.assertEqual(entries[0]['xpInPeriod'], 30, period)