Audit fixes: batch recompute, negative-baseline percentages, key-ring persistence

Three defects found reviewing the last few commits.

Deriving consumption on ingest made the batch reading endpoint quadratic. A
recompute rewrites a meter's entire consumption series, and POST
/api/v1/readings ran one per reading -- 500 readings for one meter meant 500
full rewrites. IngestByMeterAsync takes renormalize:false and the endpoint
normalizes each touched meter once after the batch.

Percentage change divided by a possibly negative baseline. A net-export meter
going from -100 to -150 exported half again as much and would have been
reported as "+50%", reading as more consumption. A non-positive baseline now
reports no basis rather than a confident lie.

The data-protection key ring had no persistent home outside Docker Compose. The
LXC installer now creates /var/lib/metervault/keys at 0700 -- the app would
otherwise create it under the default umask, leaving a key ring world-readable
-- and the Unraid template maps it, since without that every UI-entered secret
was lost whenever the container was recreated. README documents the variable
and the trust boundary: keys on disk protect against leaked database content,
not against an attacker who already has the host.

Claude-Session: https://claude.ai/code/session_01V6joyergfvVLFEizH1hJLd

tests/Integration.Tests/MeterPeriodServiceTests.cs

@@ -77,6 +77,27 @@ public sealed class MeterPeriodServiceTests(TimescaleFixture fx)
         await CleanupAsync(db, meterId);
     }
 
+    [Fact]
+    public async Task A_negative_previous_period_reports_no_basis_rather_than_an_inverted_percentage()
+    {
+        // Net export: -100 -> -150 is half again as much exported, but dividing by a negative
+        // baseline would render it "+50%", which reads as more consumption.
+        await using var db = fx.CreateContext();
+        var meterId = await SetupAsync(db, MeterMode.CumulativeCounter);
+        var today = DateOnly.FromDateTime(DateTime.UtcNow);
+        var thisMonth = new DateOnly(today.Year, today.Month, 1);
+
+        await AddConsumptionAsync(db, meterId, ConsumptionKind.Consumption, thisMonth.AddDays(1), -150);
+        await AddConsumptionAsync(db, meterId, ConsumptionKind.Consumption, thisMonth.AddMonths(-1).AddDays(3), -100);
+
+        var view = await NewService().GetAsync(meterId);
+
+        Assert.NotNull(view);
+        Assert.Null(view!.MonthChange);
+
+        await CleanupAsync(db, meterId);
+    }
+
     private MeterPeriodService NewService()
     {
         var options = Microsoft.Extensions.Options.Options.Create(