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/Costing/CostReconciliationTests.cs

@@ -36,7 +36,7 @@ public sealed class CostReconciliationTests(TimescaleFixture fx)
         });
         await db.SaveChangesAsync();
 
-        var costs = await new CostService(db).GetMeterCostsAsync(meterId, From, To);
+        var costs = await new CostService(fx).GetMeterCostsAsync(meterId, From, To);
         var computed = costs.ToDictionary(c => c.Period, c => c.Cost);
         var oracle = OracleByMonth(ReadRows(Water), dateColumn: 0, valueColumn: 4, firstDataRow: 1); // Kosten
 
@@ -64,7 +64,7 @@ public sealed class CostReconciliationTests(TimescaleFixture fx)
         db.CostCategoryMembers.Add(new CostCategoryMember { CategoryId = wasserCategory.Id, MeterId = meterId });
         await db.SaveChangesAsync();
 
-        var rollup = await new CostService(db).GetCategoryCostsAsync(wasserCategory.Id, From, To);
+        var rollup = await new CostService(fx).GetCategoryCostsAsync(wasserCategory.Id, From, To);
         var dec2022 = rollup.Single(r => r.Period == new DateOnly(2022, 12, 1));
 
         Assert.Equal(70d, dec2022.Cost, 2); // Dez 2022: 14 m³ × 5,00 €