Change JWT TTL config from duration to string for flexibility

- The mapstructure library is no longer needed for direct duration
  parsing since we now store TTLs as string durations (e.g., "15m",
  "168h") and parse them on demand via helper methods.
- This allows more flexible duration formats in configuration and moves
  the parsing responsibility to the JWT config struct itself.

internal/config/load.go

@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"strings"
 
-	"github.com/go-viper/mapstructure/v2"
 	"github.com/spf13/viper"
 )
 
@@ -24,10 +23,6 @@ func defaults(v *viper.Viper) {
 	v.SetDefault("jwt.refresh_ttl", "168h")
 }
 
-func decodeHook() viper.DecoderConfigOption {
-	return viper.DecodeHook(mapstructure.StringToTimeDurationHookFunc())
-}
-
 func New() *viper.Viper {
 	v := viper.New()
 	v.SetEnvPrefix("MYGO")
@@ -54,7 +49,7 @@ func Load(v *viper.Viper, cfgFile string) (*Config, error) {
 	}
 
 	var cfg Config
-	if err := v.Unmarshal(&cfg, decodeHook()); err != nil {
+	if err := v.Unmarshal(&cfg); err != nil {
 		return nil, fmt.Errorf("unmarshal config: %w", err)
 	}