feat: add Redis support for distributed passkey sessions

- Implement SessionStore interface for pluggable storage backends
- Add redisStore for distributed environment
- Add memoryStore as fallback for single instance
- Update router to initialize Redis client when configured
- Update .env.example with Redis documentation

backend/go.mod

@@ -27,6 +27,7 @@ require (
 	filippo.io/edwards25519 v1.1.0 // indirect
 	github.com/bytedance/sonic v1.13.2 // indirect
 	github.com/bytedance/sonic/loader v0.2.4 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/cloudwego/base64x v0.1.5 // indirect
 	github.com/dlclark/regexp2 v1.11.4 // indirect
 	github.com/fxamacker/cbor/v2 v2.8.0 // indirect
@@ -59,10 +60,12 @@ require (
 	github.com/ncruces/go-sqlite3-wasm/v2 v2.1.35300 // indirect
 	github.com/ncruces/julianday v1.0.0 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.3 // indirect
+	github.com/redis/go-redis/v9 v9.22.0 // indirect
 	github.com/spf13/pflag v1.0.6 // indirect
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.2.12 // indirect
 	github.com/x448/float16 v0.8.4 // indirect
+	go.uber.org/atomic v1.11.0 // indirect
 	golang.org/x/arch v0.16.0 // indirect
 	golang.org/x/net v0.52.0 // indirect
 	golang.org/x/sync v0.20.0 // indirect