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)

.github/workflows/docker-build.yml

@@ -8,10 +8,10 @@ name: Build and Push Docker Images
 
 on:
   push:
-    branches: [ main, develop ]
-    tags: [ 'v*.*.*' ]  # Triggered by Release Please tags
+    branches: [ main, beta ]
+    tags: [ 'v*.*.*', 'v*.*.*-beta.*' ]  # Triggered by Release Please tags (stable and beta)
   pull_request:
-    branches: [ main ]
+    branches: [ main, beta ]
   release:
     types: [ published ]  # Triggered when Release Please creates a release
   workflow_dispatch:
@@ -42,6 +42,18 @@ jobs:
     - name: Checkout code
       uses: actions/checkout@v4
 
+    - name: Determine build context
+      id: context
+      run: |
+        # Determine if this is a beta or stable release
+        if [[ "${{ github.ref }}" == refs/tags/v*-beta* ]] || [[ "${{ github.ref }}" == refs/heads/beta ]]; then
+          echo "channel=beta" >> $GITHUB_OUTPUT
+          echo "is_prerelease=true" >> $GITHUB_OUTPUT
+        else
+          echo "channel=stable" >> $GITHUB_OUTPUT
+          echo "is_prerelease=false" >> $GITHUB_OUTPUT
+        fi
+
     - name: Determine build platforms
       id: platforms
       run: |
@@ -88,10 +100,12 @@ jobs:
           type=ref,event=branch
           type=ref,event=pr
           type=semver,pattern={{version}}
-          type=semver,pattern={{major}}.{{minor}}
-          type=semver,pattern={{major}}
+          type=semver,pattern={{major}}.{{minor}},enable=${{ steps.context.outputs.is_prerelease == 'false' }}
+          type=semver,pattern={{major}},enable=${{ steps.context.outputs.is_prerelease == 'false' }}
           type=sha,format=short
           type=raw,value=latest,enable={{is_default_branch}}
+          type=raw,value=stable,enable=${{ github.ref == 'refs/heads/main' || (startsWith(github.ref, 'refs/tags/v') && steps.context.outputs.is_prerelease == 'false') }}
+          type=raw,value=beta,enable=${{ github.ref == 'refs/heads/beta' || steps.context.outputs.is_prerelease == 'true' }}
 
     - name: Build and push Backend Docker image
       uses: docker/build-push-action@v5
@@ -139,6 +153,18 @@ jobs:
     - name: Checkout code
       uses: actions/checkout@v4
 
+    - name: Determine build context
+      id: context
+      run: |
+        # Determine if this is a beta or stable release
+        if [[ "${{ github.ref }}" == refs/tags/v*-beta* ]] || [[ "${{ github.ref }}" == refs/heads/beta ]]; then
+          echo "channel=beta" >> $GITHUB_OUTPUT
+          echo "is_prerelease=true" >> $GITHUB_OUTPUT
+        else
+          echo "channel=stable" >> $GITHUB_OUTPUT
+          echo "is_prerelease=false" >> $GITHUB_OUTPUT
+        fi
+
     - name: Determine build platforms
       id: platforms
       run: |
@@ -185,10 +211,12 @@ jobs:
           type=ref,event=branch
           type=ref,event=pr
           type=semver,pattern={{version}}
-          type=semver,pattern={{major}}.{{minor}}
-          type=semver,pattern={{major}}
+          type=semver,pattern={{major}}.{{minor}},enable=${{ steps.context.outputs.is_prerelease == 'false' }}
+          type=semver,pattern={{major}},enable=${{ steps.context.outputs.is_prerelease == 'false' }}
           type=sha,format=short
           type=raw,value=latest,enable={{is_default_branch}}
+          type=raw,value=stable,enable=${{ github.ref == 'refs/heads/main' || (startsWith(github.ref, 'refs/tags/v') && steps.context.outputs.is_prerelease == 'false') }}
+          type=raw,value=beta,enable=${{ github.ref == 'refs/heads/beta' || steps.context.outputs.is_prerelease == 'true' }}
 
     - name: Build and push Frontend Docker image
       uses: docker/build-push-action@v5