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/DashboardRenderTests.cs

@@ -1,3 +1,4 @@
+using MeterVault.Infrastructure.Costing;
 using MeterVault.Infrastructure.Import;
 using MeterVault.Infrastructure.Persistence;
 using Microsoft.EntityFrameworkCore;
@@ -31,6 +32,15 @@ public sealed class DashboardRenderTests(TimescaleFixture fx)
                 Assert.Contains(await db.Meters.Select(m => m.Name).ToListAsync(), n => n == "Zähler Haus");
                 Assert.True(await db.Meters.CountAsync() >= 8);
                 Assert.True(await db.Consumption.AnyAsync());
+
+                // Regression (audit): Wasser is metered (water tariff), the Kosten Wasser column is
+                // NOT imported, so the category is not double-counted — Dez 2022 = 14 m³ × 5 € = 70 €.
+                var wasser = await db.CostCategories.FirstAsync(c => c.Name == "Wasser");
+                var rollup = await new CostService(fx).GetCategoryCostsAsync(
+                    wasser.Id,
+                    new DateTimeOffset(2022, 12, 1, 0, 0, 0, TimeSpan.Zero),
+                    new DateTimeOffset(2023, 1, 1, 0, 0, 0, TimeSpan.Zero));
+                Assert.Equal(70d, rollup.Sum(r => r.Cost), 1);
             }
 
             using var client = factory.CreateClient();