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)

README.md

@@ -79,7 +79,51 @@ Note on Docker file permissions (PUID/PGID)
 - Example in `.env`:
   - `PUID=1000`
   - `PGID=1000`
-- Without this, creating events, uploads, thumbnails, or logs can fail with “Permission denied”.
+- Without this, creating events, uploads, thumbnails, or logs can fail with "Permission denied".
+
+## 🔄 Release Channels
+
+PicPeak offers two release channels for different needs:
+
+### Stable Channel (Recommended)
+- Production-ready releases
+- Thoroughly tested before release
+- Docker tags: `stable`, `latest`, or specific version like `v2.3.0`
+
+### Beta Channel
+- Early access to new features
+- May contain bugs or incomplete functionality
+- Docker tags: `beta` or specific version like `v2.3.0-beta.1`
+
+### Switching Channels
+
+Set the `PICPEAK_CHANNEL` environment variable in your `.env` file:
+
+```bash
+# For stable releases (default)
+PICPEAK_CHANNEL=stable
+
+# For beta releases
+PICPEAK_CHANNEL=beta
+
+# For a specific version
+PICPEAK_CHANNEL=v2.3.0
+```
+
+Then update your containers:
+
+```bash
+docker-compose -f docker-compose.production.yml pull
+docker-compose -f docker-compose.production.yml up -d
+```
+
+### Update Notifications
+
+The admin dashboard automatically notifies you when updates are available for your channel. To disable update checks, set:
+
+```bash
+UPDATE_CHECK_ENABLED=false
+```
 
 ## 📖 Documentation