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/pages/admin/AdminDashboard.tsx

@@ -17,6 +17,7 @@ import { useTranslation } from 'react-i18next';
 import { useLocalizedDate } from '../../hooks/useLocalizedDate';
 
 import { Button, Card, Loading } from '../../components/common';
+import { UpdateNotification } from '../../components/admin/UpdateNotification';
 import { useQuery } from '@tanstack/react-query';
 import { eventsService } from '../../services/events.service';
 import { adminService } from '../../services/admin.service';
@@ -140,6 +141,9 @@ export const AdminDashboard: React.FC = () => {
 
   return (
     <div>
+      {/* Update Notification */}
+      <UpdateNotification />
+
       {/* Page Header */}
       <div className="flex justify-between items-center mb-8">
         <div>