refactor(server/api): writeErr accepts error; migrate admin handlers (1/3)

Rewrite writeErr(w, err) to wrap *apierror.Error via apierror.From,
preserving the existing {"error": {"code", "message"}} wire envelope.
Add writeErrWithLog helper for 500-class errors that need an operator
log line. Migrate all 13 admin_*.go handler files (~76 call sites) to
the new signature; T3 will sweep the remaining api package.

The old 4-arg writeErr is removed, so non-admin call sites in
internal/api will not compile until T3 lands. This is by design — T2
and T3 are paired.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/admin_cover_sources.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/go-chi/chi/v5"
 
+	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/coverart"
 )
 
@@ -75,7 +76,7 @@ func (h *handlers) handleUpdateCoverSource(w http.ResponseWriter, r *http.Reques
 
 	var body updateCoverSourceReq
 	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
-		writeErr(w, http.StatusBadRequest, "bad_body", "invalid JSON")
+		writeErr(w, apierror.BadRequest("bad_body", "invalid JSON"))
 		return
 	}
 
@@ -85,11 +86,11 @@ func (h *handlers) handleUpdateCoverSource(w http.ResponseWriter, r *http.Reques
 	})
 	if err != nil {
 		if errors.Is(err, coverart.ErrProviderNotFound) {
-			writeErr(w, http.StatusNotFound, "not_found", "no such provider")
+			writeErr(w, &apierror.Error{Status: http.StatusNotFound, Code: "not_found", Message: "no such provider"})
 			return
 		}
 		h.logger.Error("admin: update cover source", "id", id, "err", err)
-		writeErr(w, http.StatusInternalServerError, "server_error", "update failed")
+		writeErr(w, apierror.Internal(err))
 		return
 	}
 
@@ -141,7 +142,7 @@ func (h *handlers) handleTestCoverSource(w http.ResponseWriter, r *http.Request)
 	}
 
 	if errors.Is(err, coverart.ErrProviderNotFound) {
-		writeErr(w, http.StatusNotFound, "not_found", "no such provider")
+		writeErr(w, &apierror.Error{Status: http.StatusNotFound, Code: "not_found", Message: "no such provider"})
 		return
 	}