Return 400 for export of an unknown site

exportCtrl mapped every export failure to 500 Internal Server Error, so
requesting a backup for a non-existent site (e.g. wrong -s/--site) came
back as a misleading 500 instead of a client error — inconsistent with
the rest of the admin/public API, which returns 400 + ErrSiteNotFound
for site-lookup failures.

Add an engine.ErrSiteNotFound sentinel (wrapped at the bolt db-lookup so
the existing "site %q not found" message is unchanged) and map it to 400
+ rest.ErrSiteNotFound in exportCtrl; genuine internal failures (gzip
close/write) still return 500.

backend/app/rest/api/migrator.go

@@ -4,11 +4,13 @@ import (
 	"bytes"
 	"compress/gzip"
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
 	"os"
 	"strconv"
+	"strings"
 	"sync"
 	"time"
 
@@ -18,6 +20,7 @@ import (
 
 	"github.com/umputun/remark42/backend/app/migrator"
 	"github.com/umputun/remark42/backend/app/rest"
+	"github.com/umputun/remark42/backend/app/store/engine"
 )
 
 // Migrator rest with import and export controllers
@@ -151,7 +154,8 @@ func (m *Migrator) exportCtrl(w http.ResponseWriter, r *http.Request) {
 		var buf bytes.Buffer
 		gzWriter := gzip.NewWriter(&buf)
 		if _, err := m.NativeExporter.Export(gzWriter, siteID); err != nil {
-			rest.SendErrorJSON(w, r, http.StatusInternalServerError, err, "export failed", rest.ErrInternal)
+			code, errCode := exportErrStatus(err)
+			rest.SendErrorJSON(w, r, code, err, "export failed", errCode)
 			return
 		}
 		if err := gzWriter.Close(); err != nil {
@@ -171,10 +175,23 @@ func (m *Migrator) exportCtrl(w http.ResponseWriter, r *http.Request) {
 
 	// stream mode - write directly to response
 	if _, err := m.NativeExporter.Export(w, siteID); err != nil {
-		rest.SendErrorJSON(w, r, http.StatusInternalServerError, err, "export failed", rest.ErrInternal)
+		code, errCode := exportErrStatus(err)
+		rest.SendErrorJSON(w, r, code, err, "export failed", errCode)
 	}
 }
 
+// exportErrStatus maps an export failure to an HTTP status and error code: an unknown
+// site is a client error (400), anything else is treated as internal (500).
+// The bolt store returns the engine.ErrSiteNotFound sentinel; the rpc store loses typed
+// errors over jrpc, so the "not found" message is matched as a fallback (export only ever
+// hits a site-level lookup, so a "not found" here can only mean the site).
+func exportErrStatus(err error) (status, errCode int) {
+	if errors.Is(err, engine.ErrSiteNotFound) || strings.Contains(err.Error(), "not found") {
+		return http.StatusBadRequest, rest.ErrSiteNotFound
+	}
+	return http.StatusInternalServerError, rest.ErrInternal
+}
+
 // POST /remap?site=site-id
 // remap urls in comments based on given rules (oldUrl newUrl)
 func (m *Migrator) remapCtrl(w http.ResponseWriter, r *http.Request) {

backend/app/rest/api/migrator_test.go

@@ -391,15 +391,31 @@ func TestMigrator_Export(t *testing.T) {
 	require.Equal(t, http.StatusAccepted, resp.StatusCode)
 	waitForMigrationCompletion(t, ts)
 
-	// export wrong site, should result in error
+	// export unknown site is a client error, not internal
 	req, err = http.NewRequest("GET", ts.URL+"/api/v1/admin/export?mode=file&site=test", http.NoBody)
 	require.NoError(t, err)
 	req.SetBasicAuth("admin", "password")
 	resp, err = client.Do(req)
 	require.NoError(t, err)
+	errBody, err := io.ReadAll(resp.Body)
+	require.NoError(t, err)
 	resp.Body.Close()
-	require.Equal(t, http.StatusInternalServerError, resp.StatusCode)
+	require.Equal(t, http.StatusBadRequest, resp.StatusCode)
 	require.Equal(t, "application/json", resp.Header.Get("Content-Type"))
+	assert.Contains(t, string(errBody), `"code":6`)  // rest.ErrSiteNotFound, not ErrInternal
+	assert.Contains(t, string(errBody), `not found`) // error detail names the missing site
+
+	// unknown site in stream mode is also a client error
+	req, err = http.NewRequest("GET", ts.URL+"/api/v1/admin/export?mode=stream&site=test", http.NoBody)
+	require.NoError(t, err)
+	req.SetBasicAuth("admin", "password")
+	resp, err = client.Do(req)
+	require.NoError(t, err)
+	errBody, err = io.ReadAll(resp.Body)
+	require.NoError(t, err)
+	resp.Body.Close()
+	require.Equal(t, http.StatusBadRequest, resp.StatusCode)
+	assert.Contains(t, string(errBody), `"code":6`)
 
 	// check file mode
 	req, err = http.NewRequest("GET", ts.URL+"/api/v1/admin/export?mode=file&site=remark42", http.NoBody)

backend/app/store/engine/bolt.go

@@ -1018,7 +1018,7 @@ func (b *BoltDB) db(siteID string) (*bolt.DB, error) {
 	if res, ok := b.dbs[siteID]; ok {
 		return res, nil
 	}
-	return nil, fmt.Errorf("site %q not found", siteID)
+	return nil, fmt.Errorf("site %q %w", siteID, ErrSiteNotFound)
 }
 
 // makeRef creates reference combining url and comment id

backend/app/store/engine/engine.go

@@ -4,6 +4,7 @@ package engine
 // Includes default implementation with boltdb
 
 import (
+	"errors"
 	"sort"
 	"strings"
 	"time"
@@ -11,6 +12,10 @@ import (
 	"github.com/umputun/remark42/backend/app/store"
 )
 
+// ErrSiteNotFound is returned by engines when the requested site does not exist.
+// Its message is "not found" so wrapping it as `site %q %w` reads "site \"x\" not found".
+var ErrSiteNotFound = errors.New("not found")
+
 // NOTE: matryer/moq should be installed globally and works with `go generate ./...`
 //go:generate moq --out engine_mock.go . Interface