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.

go.mod

@@ -3,13 +3,13 @@ module github.com/dhao2001/mygo
 go 1.26.2
 
 require (
-	github.com/go-viper/mapstructure/v2 v2.4.0
 	github.com/spf13/cobra v1.10.2
 	github.com/spf13/viper v1.21.0
 )
 
 require (
 	github.com/fsnotify/fsnotify v1.9.0 // indirect
+	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
 	github.com/sagikazarmark/locafero v0.11.0 // indirect