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/config/config.go

@@ -72,6 +72,10 @@ func Load(configPath string) (*Config, error) {
 	viper.SetDefault("cache.modules.conversation_ttl", 60)
 	viper.SetDefault("cache.modules.unread_count_ttl", 30)
 	viper.SetDefault("cache.modules.group_members_ttl", 120)
+	viper.SetDefault("cache.local.enabled", true)
+	viper.SetDefault("cache.local.size", 10000)
+	viper.SetDefault("cache.local.buckets", 0)
+	viper.SetDefault("cache.local.default_ttl", 300)
 	viper.SetDefault("jwt.secret", "your-jwt-secret-key-change-in-production")
 	viper.SetDefault("jwt.access_token_expire", 86400)
 	viper.SetDefault("jwt.refresh_token_expire", 604800)