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

src/App/Api/ApiEndpoints.cs

@@ -112,6 +112,17 @@ public static class ApiEndpoints
                 .Select(s => new { s.Id, s.MeterId, Type = s.SourceType.ToString(), s.IsEnabled, s.LastSeenAt, s.LastValue, s.LastStatus })
                 .ToListAsync(ct)));
 
+        api.MapGet("/export", async (MeterVault.Infrastructure.Backup.ExportService export, CancellationToken ct) =>
+            Results.Text(await export.ExportJsonAsync(ct), "application/json"))
+            .WithSummary("Export configuration + hand-entered data as portable JSON (SDD §10).");
+
+        api.MapPost("/import", async (HttpRequest request, MeterVault.Infrastructure.Backup.ExportService export, CancellationToken ct) =>
+        {
+            using var reader = new StreamReader(request.Body);
+            await export.ImportJsonAsync(await reader.ReadToEndAsync(ct), ct);
+            return Results.Ok();
+        }).WithSummary("Restore a JSON export into an empty instance (remaps ids).");
+
         return app;
     }
 }