feat: implement beta/stable release channels with update notifications

Add dual-channel release strategy for stable and beta releases:

Release Channels:
- Stable channel: production-ready releases (stable, latest, v2.3.0)
- Beta channel: early access features (beta, v2.3.0-beta.1)
- Configurable via PICPEAK_CHANNEL environment variable

Update Notifications:
- Admin dashboard shows available updates for configured channel
- Checks GitHub Releases API with 1-hour cache
- Can be disabled with UPDATE_CHECK_ENABLED=false

CI/CD Changes:
- New release-please-beta.yml workflow for beta prereleases
- Docker build workflow produces stable/beta tags based on branch
- Beta versions use v2.3.0-beta.1 format

New Files:
- .github/workflows/release-please-beta.yml
- release-please-config-beta.json
- .release-please-manifest-beta.json
- backend/src/services/updateCheckService.js
- frontend/src/components/admin/UpdateNotification.tsx

Modified Files:
- docker-compose.production.yml (channel selection)
- .env.example (PICPEAK_CHANNEL, UPDATE_CHECK_ENABLED)
- backend/src/routes/adminSystem.js (/updates endpoint)
- frontend components (VersionInfo, AdminDashboard)
- i18n locales (en.json, de.json)
- README.md and DEPLOYMENT_GUIDE.md (documentation)

frontend/src/components/admin/UpdateNotification.tsx

@@ -0,0 +1,103 @@
+import React, { useState } from 'react';
+import { useQuery } from '@tanstack/react-query';
+import { useTranslation } from 'react-i18next';
+import { ArrowUpCircle, X, ExternalLink } from 'lucide-react';
+import { api } from '../../config/api';
+
+interface UpdateInfo {
+  enabled: boolean;
+  current: string;
+  channel: 'stable' | 'beta';
+  latest: {
+    stable: string;
+    beta: string;
+    forChannel: string;
+  };
+  updateAvailable: boolean;
+  newerBetaAvailable?: boolean;
+  lastChecked: string;
+  error?: string;
+  message?: string;
+}
+
+async function fetchUpdateInfo(): Promise<UpdateInfo> {
+  const response = await api.get<UpdateInfo>('/admin/system/updates');
+  return response.data;
+}
+
+interface UpdateNotificationProps {
+  onDismiss?: () => void;
+}
+
+export const UpdateNotification: React.FC<UpdateNotificationProps> = ({ onDismiss }) => {
+  const { t } = useTranslation();
+  const [dismissed, setDismissed] = useState(false);
+
+  const { data: updateInfo } = useQuery({
+    queryKey: ['update-check'],
+    queryFn: fetchUpdateInfo,
+    staleTime: 60 * 60 * 1000, // 1 hour
+    retry: false,
+    refetchOnWindowFocus: false
+  });
+
+  // Don't render if no update available, not enabled, or dismissed
+  if (!updateInfo?.enabled || !updateInfo?.updateAvailable || dismissed) {
+    return null;
+  }
+
+  const handleDismiss = () => {
+    setDismissed(true);
+    onDismiss?.();
+  };
+
+  const channelLabel = updateInfo.channel === 'beta'
+    ? t('admin.updates.channelBeta', 'Beta')
+    : t('admin.updates.channelStable', 'Stable');
+
+  return (
+    <div className="bg-blue-50 border-l-4 border-blue-500 p-4 mb-4 rounded-r-lg">
+      <div className="flex items-start justify-between">
+        <div className="flex items-start">
+          <ArrowUpCircle className="w-5 h-5 text-blue-500 mt-0.5 mr-3 flex-shrink-0" />
+          <div>
+            <h4 className="text-sm font-semibold text-blue-800">
+              {t('admin.updates.available', 'Update Available')}
+            </h4>
+            <p className="text-sm text-blue-700 mt-1">
+              {t('admin.updates.newVersion', 'Version {{version}} is available', {
+                version: updateInfo.latest.forChannel
+              })}
+              <span className="text-blue-500 ml-2">
+                ({t('admin.updates.currentVersion', 'Current: {{version}}', {
+                  version: updateInfo.current
+                })})
+              </span>
+            </p>
+            <p className="text-xs text-blue-600 mt-1">
+              {t('admin.updates.channel', 'Channel: {{channel}}', {
+                channel: channelLabel
+              })}
+            </p>
+            <a
+              href="https://github.com/the-luap/picpeak/releases"
+              target="_blank"
+              rel="noopener noreferrer"
+              className="inline-flex items-center text-xs text-blue-600 hover:text-blue-800 mt-2"
+            >
+              {t('admin.updates.viewReleaseNotes', 'View Release Notes')}
+              <ExternalLink className="w-3 h-3 ml-1" />
+            </a>
+          </div>
+        </div>
+        <button
+          onClick={handleDismiss}
+          className="text-blue-400 hover:text-blue-600 p-1"
+          aria-label={t('common.close', 'Close')}
+        >
+          <X className="w-4 h-4" />
+        </button>
+      </div>
+    </div>
+  );
+};