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_test.go

@@ -25,8 +25,8 @@ func TestDefaults(t *testing.T) {
 		{"database.path", cfg.Database.Path, "data/mygo.db"},
 		{"storage.driver", cfg.Storage.Driver, "local"},
 		{"storage.local.path", cfg.Storage.Local.Path, "data/files"},
-		{"jwt.access_ttl", cfg.JWT.AccessTTL, 15 * time.Minute},
-		{"jwt.refresh_ttl", cfg.JWT.RefreshTTL, 168 * time.Hour},
+		{"jwt.access_ttl", cfg.JWT.AccessTTL, "15m"},
+		{"jwt.refresh_ttl", cfg.JWT.RefreshTTL, "168h"},
 	}
 
 	for _, tt := range tests {
@@ -86,11 +86,11 @@ jwt:
 	if cfg.JWT.Secret != "test-secret" {
 		t.Errorf("jwt.secret = %q, want %q", cfg.JWT.Secret, "test-secret")
 	}
-	if cfg.JWT.AccessTTL != 30*time.Minute {
-		t.Errorf("jwt.access_ttl = %v, want %v", cfg.JWT.AccessTTL, 30*time.Minute)
+	if cfg.JWT.AccessTTL != "30m" {
+		t.Errorf("jwt.access_ttl = %q, want %q", cfg.JWT.AccessTTL, "30m")
 	}
-	if cfg.JWT.RefreshTTL != 72*time.Hour {
-		t.Errorf("jwt.refresh_ttl = %v, want %v", cfg.JWT.RefreshTTL, 72*time.Hour)
+	if cfg.JWT.RefreshTTL != "72h" {
+		t.Errorf("jwt.refresh_ttl = %q, want %q", cfg.JWT.RefreshTTL, "72h")
 	}
 }
 
@@ -195,3 +195,17 @@ func TestExplicitConfigFileNotFound(t *testing.T) {
 		t.Fatal("expected error when explicitly specifying a nonexistent config file")
 	}
 }
+
+func TestJWTConfigAccessDuration(t *testing.T) {
+	j := JWTConfig{AccessTTL: "15m"}
+	if got := j.AccessDuration(); got != 15*time.Minute {
+		t.Errorf("AccessDuration() = %v, want %v", got, 15*time.Minute)
+	}
+}
+
+func TestJWTConfigRefreshDuration(t *testing.T) {
+	j := JWTConfig{RefreshTTL: "168h"}
+	if got := j.RefreshDuration(); got != 168*time.Hour {
+		t.Errorf("RefreshDuration() = %v, want %v", got, 168*time.Hour)
+	}
+}