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/config/server.go

@@ -0,0 +1,27 @@
+package config
+
+// ServerConfig 服务器配置
+type ServerConfig struct {
+	Host string `mapstructure:"host"`
+	Port int    `mapstructure:"port"`
+	Mode string `mapstructure:"mode"`
+}
+
+// LogConfig 日志配置
+type LogConfig struct {
+	Level       string   `mapstructure:"level"`
+	Encoding    string   `mapstructure:"encoding"`
+	OutputPaths []string `mapstructure:"output_paths"`
+}
+
+// RateLimitConfig 限流配置
+type RateLimitConfig struct {
+	Enabled           bool `mapstructure:"enabled"`
+	RequestsPerMinute int  `mapstructure:"requests_per_minute"`
+}
+
+// UploadConfig 上传配置
+type UploadConfig struct {
+	MaxFileSize  int64    `mapstructure:"max_file_size"`
+	AllowedTypes []string `mapstructure:"allowed_types"`
+}