feat: add Web UI artist detail, archive, and ignore actions

Implements Task 8: artist detail page (local albums + found-missing
with ignore buttons), ignored-releases archive with restore, and POST
handlers toggling ignore flags and ignore_singles. Adds ErrArtistNotFound
sentinel so callers can distinguish missing artists, and wires routes via
the enhanced ServeMux path wildcard.

internal/database/artist_settings.go

@@ -2,6 +2,7 @@ package database
 
 import (
 	"database/sql"
+	"errors"
 	"fmt"
 )
 
@@ -17,6 +18,9 @@ func GetArtistSettings(db *DB, id string) (*ArtistSettings, error) {
 		id,
 	).Scan(&s.ID, &s.Name, &mbid, &s.IgnoreSingles, &s.IgnoreCompilations, &s.Monitored)
 	if err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			return nil, ErrArtistNotFound
+		}
 		return nil, err
 	}
 	s.MBID = mbid.String

internal/database/artist_settings_test.go

@@ -44,7 +44,7 @@ func TestGetArtistSettings_Found(t *testing.T) {
 	}
 }
 
-// TestGetArtistSettings_NotFound verifies that a missing artist returns sql.ErrNoRows.
+// TestGetArtistSettings_NotFound verifies that a missing artist returns ErrArtistNotFound.
 func TestGetArtistSettings_NotFound(t *testing.T) {
 	db, err := New(":memory:")
 	if err != nil {
@@ -53,8 +53,8 @@ func TestGetArtistSettings_NotFound(t *testing.T) {
 	defer db.Close()
 
 	_, err = GetArtistSettings(db, "nonexistent")
-	if err != sql.ErrNoRows {
-		t.Errorf("expected sql.ErrNoRows, got %v", err)
+	if err != ErrArtistNotFound {
+		t.Errorf("expected ErrArtistNotFound, got %v", err)
 	}
 }
 

internal/database/database.go

@@ -2,6 +2,7 @@ package database
 
 import (
 	"database/sql"
+	"errors"
 	"fmt"
 	"time"
 
@@ -13,6 +14,11 @@ type DB struct {
 	conn *sql.DB
 }
 
+// ErrArtistNotFound is returned by artist lookups when no row matches the given
+// ID. It is a sentinel so callers (e.g. the web UI) can distinguish "missing"
+// from other errors.
+var ErrArtistNotFound = errors.New("artist not found")
+
 // New opens a SQLite database at dbPath and runs schema migrations.
 func New(dbPath string) (*DB, error) {
 	// The _foreign_keys=on DSN parameter enables foreign key enforcement on