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_coverage.go

@@ -3,6 +3,7 @@ package api
 import (
 	"net/http"
 
+	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 )
 
@@ -24,8 +25,7 @@ func (h *handlers) handleGetLibraryCoverage(w http.ResponseWriter, r *http.Reque
 	q := dbq.New(h.pool)
 	row, err := q.GetAlbumCoverageRollup(r.Context())
 	if err != nil {
-		h.logger.Error("admin: get coverage rollup", "err", err)
-		writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
+		writeErrWithLog(w, h.logger, "admin: get coverage rollup", apierror.Internal(err))
 		return
 	}
 	writeJSON(w, http.StatusOK, coverageRollupResp{