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/ci.yml

@@ -0,0 +1,28 @@
+# Build + test on push/PR. Integration tests spin a TimescaleDB via Testcontainers, which needs
+# a Docker daemon — available on the ubuntu-latest runner.
+name: ci
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build-test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: "10.0.x"
+
+      - name: Restore
+        run: dotnet restore
+
+      - name: Build
+        run: dotnet build --no-restore -c Release
+
+      - name: Test
+        run: dotnet test --no-build -c Release --logger "trx;LogFileName=test-results.trx"