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/Costing/CostService.cs

@@ -10,24 +10,28 @@ namespace MeterVault.Infrastructure.Costing;
 /// Computes cost by joining bucketed consumption with time-ranged tariffs (SDD §7.5). Consumption
 /// is aggregated in SQL (Dapper, local-timezone buckets); the active price for each bucket is
 /// resolved in C# via <see cref="TariffResolver"/> using the month's dominant price (SDD §14.3).
-/// Categories roll up their member meters' costs plus meterless manual costs.
+/// Categories roll up their member meters' costs plus meterless manual costs. Uses a DbContext
+/// factory (short-lived context per operation) so it is safe from a Blazor circuit.
 /// </summary>
-public sealed class CostService(MeterVaultDbContext db)
+public sealed class CostService(IDbContextFactory<MeterVaultDbContext> contextFactory)
 {
-    private readonly MeterVaultDbContext _db = db;
+    private readonly IDbContextFactory<MeterVaultDbContext> _contextFactory = contextFactory;
 
     public async Task<IReadOnlyList<MeterCostBucket>> GetMeterCostsAsync(
         int meterId, DateTimeOffset from, DateTimeOffset to, CostBucket bucket = CostBucket.Month,
         CancellationToken cancellationToken = default)
     {
-        var meter = await _db.Meters.FirstOrDefaultAsync(m => m.Id == meterId, cancellationToken).ConfigureAwait(false);
+        await using var db = await _contextFactory.CreateDbContextAsync(cancellationToken).ConfigureAwait(false);
+
+        var meter = await db.Meters.AsNoTracking()
+            .FirstOrDefaultAsync(m => m.Id == meterId, cancellationToken).ConfigureAwait(false);
         if (meter is null)
         {
             return [];
         }
 
-        var tariffs = await _db.Tariffs.AsNoTracking().ToListAsync(cancellationToken).ConfigureAwait(false);
-        var series = await QueryConsumptionAsync(meterId, from, to, bucket, cancellationToken).ConfigureAwait(false);
+        var tariffs = await db.Tariffs.AsNoTracking().ToListAsync(cancellationToken).ConfigureAwait(false);
+        var series = await QueryConsumptionAsync(db, meterId, from, to, bucket, cancellationToken).ConfigureAwait(false);
 
         var results = new List<MeterCostBucket>();
         foreach (var period in series.Keys.OrderBy(k => k))
@@ -39,7 +43,8 @@ public sealed class CostService(MeterVaultDbContext db)
             var basePrice = TariffResolver.ResolveValue(tariffs, TariffComponent.BasePrice, meterId, meter.EnergyTypeId, mid);
             var feedIn = TariffResolver.ResolveValue(tariffs, TariffComponent.FeedIn, meterId, meter.EnergyTypeId, mid);
 
-            var cost = (consumption * unitPrice) + basePrice - (generation * feedIn);
+            // Base price is a monthly standing charge: prorate it to the bucket length.
+            var cost = (consumption * unitPrice) + (basePrice * MonthsInBucket(period, bucket)) - (generation * feedIn);
             results.Add(new MeterCostBucket(period, consumption, generation, cost));
         }
 
@@ -49,7 +54,9 @@ public sealed class CostService(MeterVaultDbContext db)
     public async Task<IReadOnlyList<CategoryCostBucket>> GetCategoryCostsAsync(
         int categoryId, DateTimeOffset from, DateTimeOffset to, CancellationToken cancellationToken = default)
     {
-        var members = await _db.CostCategoryMembers
+        await using var db = await _contextFactory.CreateDbContextAsync(cancellationToken).ConfigureAwait(false);
+
+        var members = await db.CostCategoryMembers.AsNoTracking()
             .Where(m => m.CategoryId == categoryId)
             .ToListAsync(cancellationToken).ConfigureAwait(false);
 
@@ -63,7 +70,7 @@ public sealed class CostService(MeterVaultDbContext db)
 
             if (member.EnergyTypeId is { } energyTypeId)
             {
-                var byType = await _db.Meters.Where(m => m.EnergyTypeId == energyTypeId).Select(m => m.Id)
+                var byType = await db.Meters.AsNoTracking().Where(m => m.EnergyTypeId == energyTypeId).Select(m => m.Id)
                     .ToListAsync(cancellationToken).ConfigureAwait(false);
                 meterIds.UnionWith(byType);
             }
@@ -78,9 +85,10 @@ public sealed class CostService(MeterVaultDbContext db)
             }
         }
 
-        var manualCosts = await _db.ManualCosts
-            .Where(c => c.CategoryId == categoryId && c.PeriodStart >= DateOnly.FromDateTime(from.UtcDateTime)
-                        && c.PeriodStart < DateOnly.FromDateTime(to.UtcDateTime))
+        var fromDate = DateOnly.FromDateTime(from.Date);
+        var toDate = DateOnly.FromDateTime(to.Date);
+        var manualCosts = await db.ManualCosts.AsNoTracking()
+            .Where(c => c.CategoryId == categoryId && c.PeriodStart >= fromDate && c.PeriodStart < toDate)
             .ToListAsync(cancellationToken).ConfigureAwait(false);
         foreach (var cost in manualCosts)
         {
@@ -91,8 +99,9 @@ public sealed class CostService(MeterVaultDbContext db)
         return [.. totals.OrderBy(kv => kv.Key).Select(kv => new CategoryCostBucket(kv.Key, kv.Value))];
     }
 
-    private async Task<Dictionary<DateOnly, (double Consumption, double Generation)>> QueryConsumptionAsync(
-        int meterId, DateTimeOffset from, DateTimeOffset to, CostBucket bucket, CancellationToken cancellationToken)
+    private static async Task<Dictionary<DateOnly, (double Consumption, double Generation)>> QueryConsumptionAsync(
+        MeterVaultDbContext db, int meterId, DateTimeOffset from, DateTimeOffset to, CostBucket bucket,
+        CancellationToken cancellationToken)
     {
         var interval = bucket switch
         {
@@ -107,7 +116,7 @@ public sealed class CostService(MeterVaultDbContext db)
             "FROM consumption WHERE meter_id = @meterId AND \"time\" >= @from AND \"time\" < @to " +
             "GROUP BY period, kind";
 
-        var connection = _db.Database.GetDbConnection();
+        var connection = db.Database.GetDbConnection();
         var command = new CommandDefinition(sql, new { meterId, from, to }, cancellationToken: cancellationToken);
         var rows = await connection.QueryAsync<ConsumptionRow>(command).ConfigureAwait(false);
 
@@ -123,6 +132,13 @@ public sealed class CostService(MeterVaultDbContext db)
         return result;
     }
 
+    private static double MonthsInBucket(DateOnly period, CostBucket bucket) => bucket switch
+    {
+        CostBucket.Day => 1.0 / DateTime.DaysInMonth(period.Year, period.Month),
+        CostBucket.Year => 12.0,
+        _ => 1.0,
+    };
+
     private static DateOnly RepresentativeDate(DateOnly period, CostBucket bucket) => bucket switch
     {
         CostBucket.Day => period,