M7: release polish — export/import, docs, easy docker push

- Easy docker push (MQTTower ergonomics + the image push it lacks): VERSION file →
  version-tag.yml (semver-guard auto-tag) → docker-publish.yml (buildx multi-arch → GHCR,
  registry centralized for one-line retarget to git.finalfactory.de) + ci.yml + build-and-push.ps1.
- Verified end to end: deploy/Dockerfile builds; docker compose stack (app + timescaledb) comes
  up healthy; /healthz and the dashboard respond in-container.
- JSON config export/import (ExportService) with id remapping on restore + GET /export, POST
  /import endpoints; round-trip test preserves meter→type, meter-scoped tariff, category links.
- README, HA/Tasmota/MQTT wiring guide (docs/wiring.md), Unraid template.
- CLAUDE.md updated to reflect the built codebase.
- i18n: locale-aware number/currency formatting (de-DE); full de UI string localization deferred.

95 tests green (56 Core + 39 integration).

Claude-Session: https://claude.ai/code/session_01WujdMtMJPbxDpDnMeK22rr

.github/workflows/docker-publish.yml

@@ -0,0 +1,50 @@
+# Builds and pushes the multi-arch MeterVault image on a vX.Y.Z tag (SDD §11).
+# Default registry is GHCR; change REGISTRY/IMAGE below (one place) to target the FinalFactory
+# registry (e.g. REGISTRY: git.finalfactory.de) and set the matching login secrets.
+name: docker-publish
+
+on:
+  push:
+    tags: ["v*"]
+  workflow_dispatch: {}
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE: ${{ github.repository }} # ghcr.io/<owner>/metervault
+
+permissions:
+  contents: read
+  packages: write
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Derive version
+        id: ver
+        run: echo "version=${GITHUB_REF_NAME#v}" >> "$GITHUB_OUTPUT"
+
+      - uses: docker/setup-qemu-action@v3
+      - uses: docker/setup-buildx-action@v3
+
+      - name: Log in to the registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: deploy/Dockerfile
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: |
+            ${{ env.REGISTRY }}/${{ env.IMAGE }}:${{ steps.ver.outputs.version }}
+            ${{ env.REGISTRY }}/${{ env.IMAGE }}:latest
+          cache-from: type=gha
+          cache-to: type=gha,mode=max