add thumbnail field to album model, update album retrieval to use POST method, and enhance album list component with dynamic fetching

backend/internal/models/album.go

@@ -15,6 +15,7 @@ type Album struct {
 	// Public albums have a default access level of 0 for all visitors, including guests.
 	// Private albums require a user with access to be logged in to view, or a magic link to be used.
 	ParentID  string `gorm:"not null"` // The ID of the parent album, if any. This is an empty string for root albums.
+	Thumbnail string `gorm:"not null"` // The media ID of the thumbnail for the album.
 	CreatedAt time.Time
 	UpdatedAt time.Time
 }