fix(email): fail-fast IMAP timeouts + manual 'Check now' poll

- Root cause of the 502s: ImapFlow had no connect timeout, so a wrong host/port
  (e.g. IMAP on an SMTP port) hung the request until the proxy returned 502 with
  no message. Added connectionTimeout/greetingTimeout/socketTimeout + a hard
  connectWithTimeout() race on every IMAP client (detect/test/roundtrip/poll).
- Error routes now return 422 with the underlying reason (was 502, which
  collided with the proxy's own 502 and hid the message).
- New 'Check now' button + POST /incoming-config/poll runs the poller on demand
  (respects the incomingMail flag) and reports disabled/unconfigured/busy or N
  ingested — so 'nothing in Received' is diagnosable without waiting 60s.
- en/de strings

frontend/src/components/admin/IncomingMailConfigCard.tsx

@@ -11,7 +11,7 @@ import React, { useEffect, useState } from 'react';
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import { useTranslation } from 'react-i18next';
 import { toast } from 'react-toastify';
-import { Save, Server, User, Lock, Eye, EyeOff, FolderSearch, PlugZap, Mailbox } from 'lucide-react';
+import { Save, Server, User, Lock, Eye, EyeOff, FolderSearch, PlugZap, Mailbox, RefreshCw } from 'lucide-react';
 import { Button, Card, Input, Loading } from '../common';
 import { emailService, type IncomingMailConfig, type ImapFolder } from '../../services/email.service';
 
@@ -56,6 +56,23 @@ export const IncomingMailConfigCard: React.FC = () => {
     onError: (e: any) => toast.error(e?.response?.data?.error || e.message || t('email.incoming.roundTripFailed', 'Round-trip test failed.')),
   });
 
+  const poll = useMutation({
+    mutationFn: () => emailService.pollIncoming(),
+    onSuccess: (r) => {
+      if (r.skipped === 'disabled') {
+        toast.info(t('email.incoming.pollDisabled', 'Incoming mail is turned off — enable it under Settings → Features.'));
+      } else if (r.skipped === 'unconfigured') {
+        toast.info(t('email.incoming.pollUnconfigured', 'Save the incoming mail settings first.'));
+      } else if (r.skipped === 'busy') {
+        toast.info(t('email.incoming.pollBusy', 'A poll is already running — try again in a moment.'));
+      } else {
+        toast.success(t('email.incoming.pollOk', 'Checked mailbox — {{count}} new email(s) ingested.', { count: r.processed || 0 }));
+        qc.invalidateQueries({ queryKey: ['received-emails'] });
+      }
+    },
+    onError: (e: any) => toast.error(e?.response?.data?.error || e.message || t('email.incoming.pollFailed', 'Mailbox poll failed.')),
+  });
+
   const detect = useMutation({
     mutationFn: () => emailService.listIncomingFolders(cfg),
     onSuccess: (list) => {
@@ -186,6 +203,17 @@ export const IncomingMailConfigCard: React.FC = () => {
           >
             {t('email.incoming.roundTrip', 'Round-trip test')}
           </Button>
+          <Button
+            variant="outline"
+            onClick={() => poll.mutate()}
+            isLoading={poll.isPending}
+            disabled={!cfg.imap_host || !cfg.imap_user}
+            leftIcon={<RefreshCw className="w-5 h-5" />}
+            className="whitespace-nowrap"
+            title={t('email.incoming.pollHint', 'Check the mailbox now instead of waiting for the 60-second poll. Ingests unread attachments into Incoming invoices.') as string}
+          >
+            {t('email.incoming.poll', 'Check now')}
+          </Button>
           <Button variant="primary" onClick={() => save.mutate()} isLoading={save.isPending} leftIcon={<Save className="w-5 h-5" />} className="flex-1 min-w-[12rem]">
             {t('email.incoming.save', 'Save Incoming Mail Settings')}
           </Button>