Cast oldest[0][1] to float in RedisSlidingWindow fallback

mypy complained: 'Unsupported operand types for - ("str" and "float")'
on rate_limit.py:87. redis-py's zrange withscores=True returns the
score as whatever the codec produces (often str), but we treat it as
a numeric millisecond timestamp. Cast to float before subtracting
the cutoff.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/cartsnitch_api/middleware/rate_limit.py

@@ -84,7 +84,7 @@ class RedisSlidingWindow:
             if current_count >= self.max_requests:
                 oldest = await self.redis.zrange(key, 0, 0, withscores=True)
                 if oldest:
-                    retry_after = int((oldest[0][1] - cutoff) / 1000) + 1
+                    retry_after = int((float(oldest[0][1]) - cutoff) / 1000) + 1
                 else:
                     retry_after = self.window_seconds
                 return False, 0, retry_after