refactor: introduce Wire dependency injection and interface-based architecture

- Add Google Wire for compile-time dependency injection
- Replace concrete service types with interfaces across handlers
- Remove global state (DB, cache) in favor of constructor injection
- Split monolithic files into focused modules:
  - config: separate files for each config domain
  - dto: converters split by domain (user, post, message, group)
  - cache: separate metrics.go and redis_cache.go
- Introduce unified apperrors package with string-based error codes
- Add transaction support with context-aware repository methods
- Upgrade golang.org/x/crypto from 0.17.0 to 0.26.0

internal/model/init.go

@@ -14,11 +14,8 @@ import (
 	"carrot_bbs/internal/config"
 )
 
-// DB 全局数据库连接
-var DB *gorm.DB
-
-// InitDB 初始化数据库连接
-func InitDB(cfg *config.DatabaseConfig) error {
+// NewDB 创建数据库连接（用于 Wire 依赖注入）
+func NewDB(cfg *config.DatabaseConfig) (*gorm.DB, error) {
 	var err error
 	var db *gorm.DB
 	gormLogger := logger.New(
@@ -52,28 +49,26 @@ func InitDB(cfg *config.DatabaseConfig) error {
 	}
 
 	if err != nil {
-		return fmt.Errorf("failed to connect to database: %w", err)
+		return nil, fmt.Errorf("failed to connect to database: %w", err)
 	}
 
-	DB = db
-
 	// 配置连接池（SQLite不支持连接池配置，跳过）
 	if cfg.Type != "sqlite" {
-		sqlDB, err := DB.DB()
+		sqlDB, err := db.DB()
 		if err != nil {
-			return fmt.Errorf("failed to get database instance: %w", err)
+			return nil, fmt.Errorf("failed to get database instance: %w", err)
 		}
 		sqlDB.SetMaxIdleConns(cfg.MaxIdleConns)
 		sqlDB.SetMaxOpenConns(cfg.MaxOpenConns)
 	}
 
 	// 自动迁移
-	if err := autoMigrate(DB); err != nil {
-		return fmt.Errorf("failed to auto migrate: %w", err)
+	if err := autoMigrate(db); err != nil {
+		return nil, fmt.Errorf("failed to auto migrate: %w", err)
 	}
 
 	log.Printf("Database connected (%s) and migrated successfully", cfg.Type)
-	return nil
+	return db, nil
 }
 
 func parseGormLogLevel(level string) logger.LogLevel {
@@ -155,8 +150,8 @@ func autoMigrate(db *gorm.DB) error {
 }
 
 // CloseDB 关闭数据库连接
-func CloseDB() {
-	if sqlDB, err := DB.DB(); err == nil {
+func CloseDB(db *gorm.DB) {
+	if sqlDB, err := db.DB(); err == nil {
 		sqlDB.Close()
 	}
 }