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/config.go

@@ -34,9 +34,19 @@ type LocalStorageConfig struct {
 }
 
 type JWTConfig struct {
-	Secret     string        `mapstructure:"secret"`
-	AccessTTL  time.Duration `mapstructure:"access_ttl"`
-	RefreshTTL time.Duration `mapstructure:"refresh_ttl"`
+	Secret     string `mapstructure:"secret"`
+	AccessTTL  string `mapstructure:"access_ttl"`
+	RefreshTTL string `mapstructure:"refresh_ttl"`
+}
+
+func (j JWTConfig) AccessDuration() time.Duration {
+	d, _ := time.ParseDuration(j.AccessTTL)
+	return d
+}
+
+func (j JWTConfig) RefreshDuration() time.Duration {
+	d, _ := time.ParseDuration(j.RefreshTTL)
+	return d
 }
 
 func (c *Config) Validate() error {
@@ -62,12 +72,12 @@ func (c *Config) Validate() error {
 		errs = append(errs, errors.New("jwt.secret: must not be empty"))
 	}
 
-	if c.JWT.AccessTTL <= 0 {
-		errs = append(errs, fmt.Errorf("jwt.access_ttl: %v must be positive", c.JWT.AccessTTL))
+	if _, err := time.ParseDuration(c.JWT.AccessTTL); err != nil {
+		errs = append(errs, fmt.Errorf("jwt.access_ttl: %w", err))
 	}
 
-	if c.JWT.RefreshTTL <= 0 {
-		errs = append(errs, fmt.Errorf("jwt.refresh_ttl: %v must be positive", c.JWT.RefreshTTL))
+	if _, err := time.ParseDuration(c.JWT.RefreshTTL); err != nil {
+		errs = append(errs, fmt.Errorf("jwt.refresh_ttl: %w", err))
 	}
 
 	return errors.Join(errs...)