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/repository/transaction.go

@@ -0,0 +1,43 @@
+package repository
+
+import (
+	"context"
+
+	"gorm.io/gorm"
+)
+
+// TransactionManager 事务管理器接口
+type TransactionManager interface {
+	// RunInTransaction 在事务中执行函数
+	RunInTransaction(ctx context.Context, fn func(ctx context.Context) error) error
+}
+
+// gormTransactionManager GORM 事务管理器实现
+type gormTransactionManager struct {
+	db *gorm.DB
+}
+
+// NewTransactionManager 创建事务管理器
+func NewTransactionManager(db *gorm.DB) TransactionManager {
+	return &gormTransactionManager{db: db}
+}
+
+// RunInTransaction 在事务中执行函数
+func (m *gormTransactionManager) RunInTransaction(ctx context.Context, fn func(ctx context.Context) error) error {
+	return m.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
+		// 将事务 TX 存入 context
+		ctxWithTx := context.WithValue(ctx, txKey{}, tx)
+		return fn(ctxWithTx)
+	})
+}
+
+// txKey 事务 context key
+type txKey struct{}
+
+// GetTxFromContext 从 context 获取事务 TX
+func GetTxFromContext(ctx context.Context) *gorm.DB {
+	if tx, ok := ctx.Value(txKey{}).(*gorm.DB); ok {
+		return tx
+	}
+	return nil
+}