feat: add affinity scheduling controls

backend/internal/repository/lua/update_affinity.lua

@@ -1,11 +1,11 @@
 -- 原子双写正向+反向索引
--- KEYS[1] = client_affinity:{groupID}:{clientID}       (正向: client → accounts)
--- KEYS[2] = client_affinity_rev:{groupID}:{accountID}  (反向: account → clients)
+-- KEYS[1] = affinity:{groupID}:{userID}/{clientID}    (正向: member → accounts)
+-- KEYS[2] = affinity_rev:{groupID}:{accountID}        (反向: account → members)
 -- ARGV[1] = 当前时间戳 (score)
 -- ARGV[2] = TTL 秒数
 -- ARGV[3] = accountID (正向索引的成员)
 -- ARGV[4] = 过期阈值时间戳 (now - ttl)
--- ARGV[5] = clientID  (反向索引的成员)
+-- ARGV[5] = {userID}/{clientID} (反向索引的成员)
 redis.call('ZREMRANGEBYSCORE', KEYS[1], '-inf', ARGV[4])
 redis.call('ZADD', KEYS[1], ARGV[1], ARGV[3])
 redis.call('EXPIRE', KEYS[1], ARGV[2])