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_scan.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/jackc/pgx/v5"
 
+	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
 )
@@ -39,8 +40,7 @@ func (h *handlers) handleGetScanStatus(w http.ResponseWriter, r *http.Request) {
 			writeJSON(w, http.StatusOK, scanStatusResp{InFlight: false})
 			return
 		}
-		h.logger.Error("admin: get scan status", "err", err)
-		writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
+		writeErrWithLog(w, h.logger, "admin: get scan status", apierror.Internal(err))
 		return
 	}
 
@@ -85,8 +85,7 @@ func (h *handlers) handleTriggerScan(w http.ResponseWriter, _ *http.Request) {
 		h.logger.With("source", "manual"), h.scanCfg,
 	)
 	if err != nil {
-		h.logger.Error("admin: trigger scan failed", "err", err)
-		writeErr(w, http.StatusInternalServerError, "server_error", "trigger failed")
+		writeErrWithLog(w, h.logger, "admin: trigger scan failed", apierror.Internal(err))
 		return
 	}
 	if !started {