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

src/Infrastructure/DependencyInjection.cs

@@ -15,11 +15,16 @@ public static class DependencyInjection
         this IServiceCollection services,
         string connectionString)
     {
-        services.AddDbContext<MeterVaultDbContext>(options =>
+        // A factory (for per-operation contexts in Blazor components/read services — a shared
+        // circuit-scoped DbContext is not thread-safe) plus a scoped context (from the factory)
+        // for request/tick-scoped services (API, workers, importers) that inject it directly.
+        services.AddDbContextFactory<MeterVaultDbContext>(options =>
             options
                 .UseNpgsql(connectionString, npgsql =>
                     npgsql.MigrationsAssembly(typeof(MeterVaultDbContext).Assembly.FullName))
                 .UseSnakeCaseNamingConvention());
+        services.AddScoped<MeterVaultDbContext>(sp =>
+            sp.GetRequiredService<IDbContextFactory<MeterVaultDbContext>>().CreateDbContext());
 
         services.AddSingleton<INormalizationEngine>(_ => NormalizationEngine.CreateDefault());
         services.AddScoped<NormalizationService>();