feat: add hook system, QR code login, and layered cache

- Implement extensible hook system for content moderation with
  builtin and AI-powered moderation hooks
- Add QR code login feature with SSE for real-time status updates
  and scan/confirm/cancel workflow
- Introduce layered cache with local LRU + Redis backend for
  improved read performance
- Refactor post and comment services to use hook-based moderation
  instead of direct AI service calls
- Add local cache configuration options (size, buckets, ttl)

go.mod

@@ -6,6 +6,7 @@ require (
 	github.com/alicebob/miniredis/v2 v2.37.0
 	github.com/casbin/casbin/v3 v3.10.0
 	github.com/casbin/gorm-adapter/v3 v3.41.0
+	github.com/cespare/xxhash/v2 v2.3.0
 	github.com/disintegration/imaging v1.6.2
 	github.com/gin-gonic/gin v1.12.0
 	github.com/golang-jwt/jwt/v5 v5.3.1
@@ -33,7 +34,6 @@ require (
 	github.com/bytedance/sonic v1.15.0 // indirect
 	github.com/bytedance/sonic/loader v0.5.0 // indirect
 	github.com/casbin/govaluate v1.10.0 // indirect
-	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/cloudwego/base64x v0.1.6 // indirect
 	github.com/dgryski/go-rendezvous v0.0.0-20200823014737-9f7001d12a5f // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect