fix: show artist covers (raw url in artist info, cover id elsewhere) via scanned guessed artist folder

closes #180
closes #179

server/db/model.go

@@ -43,11 +43,13 @@ func joinInt(in []int, sep string) string {
 }
 
 type Artist struct {
-	ID         int      `gorm:"primary_key"`
-	Name       string   `gorm:"not null; unique_index"`
-	NameUDec   string   `sql:"default: null"`
-	Albums     []*Album `gorm:"foreignkey:TagArtistID"`
-	AlbumCount int      `sql:"-"`
+	ID              int      `gorm:"primary_key"`
+	Name            string   `gorm:"not null; unique_index"`
+	NameUDec        string   `sql:"default: null"`
+	Albums          []*Album `gorm:"foreignkey:TagArtistID"`
+	AlbumCount      int      `sql:"-"`
+	GuessedFolder   *Album
+	GuessedFolderID int `sql:"default: null; type:int REFERENCES albums(id)"`
 }
 
 func (a *Artist) SID() *specid.ID {