feat(backend): 限流/登录失败锁状态外置 Redis（todo #2）

新包 internal/ratelimit：内存/redis(GCRA+Lua) 双实现 + 出错逐调用降级内存
（fail-open 到内存不 fail-closed）。REDIS_ADDR 空=内存模式，行为与既往一致；
配置后跨重启保状态、支持多实例。miniredis 全覆盖测试，零真实外部依赖。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/go.sum

@@ -1,11 +1,15 @@
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@@ -31,6 +35,8 @@ github.com/go-playground/universal-translator v0.18.1 h1:Bcnm0ZwsGyWbCzImXv+pAJn
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+github.com/go-redis/redis_rate/v10 v10.0.1/go.mod h1:EMiuO9+cjRkR7UvdvwMO7vbgqJkltQHtwbdIQvaBKIU=
 github.com/go-sql-driver/mysql v1.8.1 h1:LedoTUt/eveggdHS9qUFC1EFSa8bU2+1pZjSRpvNJ1Y=
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 github.com/go-viper/mapstructure/v2 v2.4.0 h1:EBsztssimR/CONLSZZ04E8qAkxNYq4Qp9LvH92wZUgs=
@@ -81,6 +87,8 @@ github.com/quic-go/qpack v0.6.0 h1:g7W+BMYynC1LbYLSqRt8PBg5Tgwxn214ZZR34VIOjz8=
 github.com/quic-go/qpack v0.6.0/go.mod h1:lUpLKChi8njB4ty2bFLX2x4gzDqXwUpaO1DP9qMDZII=
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@@ -128,8 +136,12 @@ github.com/xuri/excelize/v2 v2.10.1 h1:V62UlqopMqha3kOpnlHy2CcRVw1V8E63jFoWUmMzx
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+go.uber.org/atomic v1.11.0 h1:ZvwS0R+56ePWxUNi+Atn9dWONBPp/AUETXlHW0DxSjE=
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