fix: use sentinel ErrLocationNotFound to satisfy errorlint

Replace errors.New("location not found") string with a package-level
sentinel var so callers can use errors.Is() instead of string comparison,
which errorlint flags as unsafe.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

internal/ghl/admin.go

@@ -3,6 +3,7 @@ package ghl
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"io"
 	"log/slog"
 	"net/http"
@@ -153,7 +154,7 @@ func (h *AdminHandler) HandleSetLocationConfig(w http.ResponseWriter, r *http.Re
 
 	if err := h.store.UpdateLocationConfig(ctx, locationID, payload.SenderID, payload.CastAPIKey); err != nil {
 		slog.Error("admin: update location config failed", "location_id", locationID, "err", err)
-		if err.Error() == "location not found" {
+		if errors.Is(err, store.ErrLocationNotFound) {
 			http.Error(w, "location not found", http.StatusNotFound)
 			return
 		}

internal/store/mongo.go

@@ -10,6 +10,9 @@ import (
 	"go.mongodb.org/mongo-driver/v2/mongo/options"
 )
 
+// ErrLocationNotFound is returned when a location ID has no stored token record.
+var ErrLocationNotFound = errors.New("location not found")
+
 type TokenRecord struct {
 	LocationID   string    `bson:"location_id"`
 	CompanyID    string    `bson:"company_id"`
@@ -99,7 +102,7 @@ func (s *Store) UpdateLocationConfig(ctx context.Context, locationID, senderID,
 		return err
 	}
 	if res.MatchedCount == 0 {
-		return errors.New("location not found")
+		return ErrLocationNotFound
 	}
 	return nil
 }