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/MeterVaultAppFactory.cs

@@ -7,7 +7,7 @@ namespace MeterVault.Integration.Tests;
 /// Boots the real ASP.NET Core app in-memory against the shared Timescale container.
 /// Migrations are already applied by <see cref="TimescaleFixture"/>, so startup migration is off.
 /// </summary>
-public sealed class MeterVaultAppFactory(string connectionString) : WebApplicationFactory<Program>
+public sealed class MeterVaultAppFactory(string connectionString, bool configureApiKey = true) : WebApplicationFactory<Program>
 {
     public const string ApiKey = "test-api-key";
 
@@ -17,6 +17,9 @@ public sealed class MeterVaultAppFactory(string connectionString) : WebApplicati
         builder.UseSetting("ConnectionStrings:Default", connectionString);
         builder.UseSetting("MeterVault:RunMigrationsAtStartup", "false");
         builder.UseSetting("MeterVault:EnableLiveIngestion", "false");
-        builder.UseSetting("MeterVault:ApiKeys:0", ApiKey);
+        if (configureApiKey)
+        {
+            builder.UseSetting("MeterVault:ApiKeys:0", ApiKey);
+        }
     }
 }