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/database.go

@@ -0,0 +1,39 @@
+package config
+
+import "fmt"
+
+// DatabaseConfig 数据库配置
+type DatabaseConfig struct {
+	Type                 string         `mapstructure:"type"`
+	SQLite               SQLiteConfig   `mapstructure:"sqlite"`
+	Postgres             PostgresConfig `mapstructure:"postgres"`
+	MaxIdleConns         int            `mapstructure:"max_idle_conns"`
+	MaxOpenConns         int            `mapstructure:"max_open_conns"`
+	LogLevel             string         `mapstructure:"log_level"`
+	SlowThresholdMs      int            `mapstructure:"slow_threshold_ms"`
+	IgnoreRecordNotFound bool           `mapstructure:"ignore_record_not_found"`
+	ParameterizedQueries bool           `mapstructure:"parameterized_queries"`
+}
+
+// SQLiteConfig SQLite 配置
+type SQLiteConfig struct {
+	Path string `mapstructure:"path"`
+}
+
+// PostgresConfig PostgreSQL 配置
+type PostgresConfig struct {
+	Host     string `mapstructure:"host"`
+	Port     int    `mapstructure:"port"`
+	User     string `mapstructure:"user"`
+	Password string `mapstructure:"password"`
+	DBName   string `mapstructure:"dbname"`
+	SSLMode  string `mapstructure:"sslmode"`
+}
+
+// DSN 返回 PostgreSQL 连接字符串
+func (d PostgresConfig) DSN() string {
+	return fmt.Sprintf(
+		"host=%s port=%d user=%s password=%s dbname=%s sslmode=%s",
+		d.Host, d.Port, d.User, d.Password, d.DBName, d.SSLMode,
+	)
+}