refactor(server): final writeErr migration + ErrNotFound aliases (T3c)

internal/coverart/provider.go

@@ -5,6 +5,8 @@ import (
 	"errors"
 	"fmt"
 	"sync"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
 )
 
 // Provider is the base shape every cover-art source implements.
@@ -112,7 +114,11 @@ var ErrProviderNotFound = errors.New("coverart: provider not registered")
 // this MBID — a terminal-this-pass result. The enricher treats this
 // as "try the next provider in the chain"; if all providers return
 // ErrNotFound, the row settles to cover_art_source='none'.
-var ErrNotFound = errors.New("coverart: not found")
+//
+// Aliased to apierror.ErrNotFound so handler error mapping (errors.Is
+// against the shared sentinel) and existing per-package callsites both
+// resolve to the same pointer.
+var ErrNotFound = apierror.ErrNotFound
 
 // ErrTransient indicates a temporary failure (5xx, network, timeout,
 // auth issue). The enricher leaves the row's source NULL so the next