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)

internal/cache/cache.go

@@ -4,11 +4,14 @@ import (
 	"cmp"
 	"context"
 	"encoding/json"
+	"errors"
 	"math/rand"
 	"sync"
 	"time"
 )
 
+var ErrKeyNotFound = errors.New("key not found")
+
 // Cache 缓存接口
 type Cache interface {
 	// Set 设置缓存值，支持TTL