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)

docker-compose.production.yml

@@ -38,7 +38,8 @@ services:
 
   backend:
     # Use pre-built image from GitHub Container Registry
-    image: ghcr.io/the-luap/picpeak/backend:latest
+    # PICPEAK_CHANNEL: 'stable' (default), 'beta', or specific version like 'v2.3.0'
+    image: ghcr.io/the-luap/picpeak/backend:${PICPEAK_CHANNEL:-stable}
     container_name: picpeak-backend
     env_file: .env
     environment:
@@ -46,6 +47,7 @@ services:
       - DB_HOST=${DB_HOST:-postgres}
       - REDIS_HOST=redis
       - PHOTOS_DIR=/app/storage/events
+      - PICPEAK_RELEASE_CHANNEL=${PICPEAK_CHANNEL:-stable}
     volumes:
       - ${APP_STORAGE}:/app/storage
       - ${LOGS}:/app/logs
@@ -69,7 +71,8 @@ services:
 
   frontend:
     # Use pre-built image from GitHub Container Registry
-    image: ghcr.io/the-luap/picpeak/frontend:latest
+    # Uses same channel as backend for consistency
+    image: ghcr.io/the-luap/picpeak/frontend:${PICPEAK_CHANNEL:-stable}
     container_name: picpeak-frontend
     # Note: Pre-built frontend uses Nginx to proxy /api to backend:3001.
     # Prefer keeping API base as '/api' in builds to avoid CORS.