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/Backup/ExportService.cs

@@ -118,6 +118,7 @@ public sealed class ExportService(MeterVaultDbContext db)
             cost.Id = 0;
             cost.CategoryId = cost.CategoryId is { } cid ? categoryMap.GetValueOrDefault(cid, cid) : null;
             cost.MeterId = cost.MeterId is { } mid ? meterMap.GetValueOrDefault(mid, mid) : null;
+            cost.ImportBatchId = null; // the original import batch isn't part of the export
             _db.ManualCosts.Add(cost);
         }
 
@@ -125,6 +126,7 @@ public sealed class ExportService(MeterVaultDbContext db)
         {
             meterEvent.Id = 0;
             meterEvent.MeterId = meterMap.GetValueOrDefault(meterEvent.MeterId, meterEvent.MeterId);
+            meterEvent.ImportBatchId = null;
             _db.MeterEvents.Add(meterEvent);
         }