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.mod

@@ -22,9 +22,11 @@ require (
 
 require (
 	filippo.io/edwards25519 v1.1.0 // indirect
+	github.com/alicebob/miniredis/v2 v2.38.0 // indirect
 	github.com/bytedance/gopkg v0.1.3 // indirect
 	github.com/bytedance/sonic v1.15.0 // indirect
 	github.com/bytedance/sonic/loader v0.5.0 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/cloudwego/base64x v0.1.6 // indirect
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/fsnotify/fsnotify v1.9.0 // indirect
@@ -33,6 +35,7 @@ require (
 	github.com/go-playground/locales v0.14.1 // indirect
 	github.com/go-playground/universal-translator v0.18.1 // indirect
 	github.com/go-playground/validator/v10 v10.30.1 // indirect
+	github.com/go-redis/redis_rate/v10 v10.0.1 // indirect
 	github.com/go-sql-driver/mysql v1.8.1 // indirect
 	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
 	github.com/goccy/go-json v0.10.5 // indirect
@@ -51,6 +54,7 @@ require (
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/quic-go/qpack v0.6.0 // indirect
 	github.com/quic-go/quic-go v0.59.0 // indirect
+	github.com/redis/go-redis/v9 v9.21.0 // indirect
 	github.com/richardlehane/mscfb v1.0.6 // indirect
 	github.com/richardlehane/msoleps v1.0.6 // indirect
 	github.com/sagikazarmark/locafero v0.11.0 // indirect
@@ -64,7 +68,9 @@ require (
 	github.com/ugorji/go/codec v1.3.1 // indirect
 	github.com/xuri/efp v0.0.1 // indirect
 	github.com/xuri/nfp v0.0.2-0.20250530014748-2ddeb826f9a9 // indirect
+	github.com/yuin/gopher-lua v1.1.1 // indirect
 	go.mongodb.org/mongo-driver/v2 v2.5.0 // indirect
+	go.uber.org/atomic v1.11.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/arch v0.22.0 // indirect
 	golang.org/x/image v0.25.0 // indirect