Polish/audit: fix bugs found by 3 subsystem audits

Correctness/data:
- Fix demo cost double-count: reference importer no longer imports the Kosten Strom/Wasser
  columns for categories that are metered (only Heizung), so Wasser rollup is 70€ not 140€.
- Spurious-decrease guard: only a reset/swap in the window (prevReading, thisReading] explains
  a decrease — an old historical reset no longer permanently disables the guard.
- Gate swap auto-detection on MappingProfile.DetectCumulativeSwaps (flag was ignored).
- Prorate basePrice by bucket length (day/month/year); guard virtual expressions against NaN/Inf.

Concurrency/infra:
- Blazor: register a DbContextFactory; CostService/DashboardService and the read pages now use
  short-lived per-operation contexts (no shared circuit DbContext); guard Trends re-entrancy.
- /events: wrap event insert + consumption recompute in one transaction (atomic); 404 (not 500)
  on unknown meter.
- MQTT worker: subscribe to newly-added topics on each tick; move client cleanup into finally.
- Migrations: CREATE MATERIALIZED VIEW IF NOT EXISTS + if_not_exists on CAgg/compression/
  hypertable calls (re-run-safe after a mid-migration crash).
- HA worker: prune stale poll-schedule entries; export: null dangling ImportBatchIds on restore.

API/security:
- API fail-closed by default: with no keys and AllowAnonymousApi off, /api/v1 returns 401
  (protects /export and /import). New MeterVault:AllowAnonymousApi opt-in.
- Cap /readings batch at 5000; report ignored (unknown-meter) count; enums as strings in JSON.

+4 regression tests (guard window, API closed, /events 404, no demo double-count). 98 tests
green; Docker deploy re-verified healthy with the API fail-closed.

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

tests/Integration.Tests/ApiTests.cs

@@ -52,6 +52,37 @@ public sealed class ApiTests(TimescaleFixture fx)
         }
     }
 
+    private sealed record EventPush(int MeterId, DateTimeOffset Time, string Type,
+        double? Amount, double? PrevValue, double? NewValue, string? Unit, string? Notes);
+
+    [Fact]
+    public async Task Api_is_closed_when_no_keys_are_configured()
+    {
+        using var factory = new MeterVaultAppFactory(fx.ConnectionString, configureApiKey: false);
+        using var client = factory.CreateClient();
+
+        var response = await client.PostAsJsonAsync("/api/v1/readings",
+            new[] { new ReadingPush(1, DateTimeOffset.UtcNow, 1) });
+
+        Assert.Equal(HttpStatusCode.Unauthorized, response.StatusCode);
+    }
+
+    [Fact]
+    public async Task Events_for_a_missing_meter_return_404_not_500()
+    {
+        using var factory = new MeterVaultAppFactory(fx.ConnectionString);
+        using var client = factory.CreateClient();
+
+        using var request = new HttpRequestMessage(HttpMethod.Post, "/api/v1/events")
+        {
+            Content = JsonContent.Create(new EventPush(999999, DateTimeOffset.UtcNow, "Delivery", 100, null, null, "L", null)),
+        };
+        request.Headers.Add("X-Api-Key", MeterVaultAppFactory.ApiKey);
+        var response = await client.SendAsync(request);
+
+        Assert.Equal(HttpStatusCode.NotFound, response.StatusCode);
+    }
+
     [Fact]
     public async Task Meters_endpoint_and_swagger_are_available()
     {