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/email.go

@@ -0,0 +1,15 @@
+package config
+
+// EmailConfig 邮件配置
+type EmailConfig struct {
+	Enabled            bool   `mapstructure:"enabled"`
+	Host               string `mapstructure:"host"`
+	Port               int    `mapstructure:"port"`
+	Username           string `mapstructure:"username"`
+	Password           string `mapstructure:"password"`
+	FromAddress        string `mapstructure:"from_address"`
+	FromName           string `mapstructure:"from_name"`
+	UseTLS             bool   `mapstructure:"use_tls"`
+	InsecureSkipVerify bool   `mapstructure:"insecure_skip_verify"`
+	Timeout            int    `mapstructure:"timeout"`
+}