M0: scaffold solution, EF+Timescale schema, /healthz

Five-project Clean Architecture solution (Core/Infrastructure/App + Core.Tests/
Integration.Tests) on .NET 10 with central package management, snake_case EF mapping,
and shared build/style config.

- Full domain entity set + EF DbContext for the SDD §5.3 schema (singular table names).
- InitialSchema migration (relational) + TimescaleHypertables migration (raw SQL:
  create_hypertable + compression on reading, hypertable on consumption).
- App wiring: Serilog (actually wired, unlike MQTTower), DbContext, migrate-on-startup,
  /healthz. Serves plain HTTP behind a reverse proxy (no HTTPS redirect).
- deploy/Dockerfile (2-stage, ICU-capable) + docker-compose (app + timescaledb).
- Integration.Tests: shared TimescaleFixture (Testcontainers) — migrations, hypertables,
  compression policy, and /healthz all verified green (4/4).

Claude-Session: https://claude.ai/code/session_01WujdMtMJPbxDpDnMeK22rr

src/Core/Domain/Reading.cs

@@ -0,0 +1,24 @@
+namespace MeterVault.Core.Domain;
+
+/// <summary>
+/// A raw, immutable, timestamped meter value — the audit truth (SDD §5.3 <c>reading</c>).
+/// Backed by a TimescaleDB hypertable with composite key (meter_id, time).
+/// The value is a register / level / hours / rate in the meter's own unit.
+/// </summary>
+public sealed class Reading
+{
+    public DateTimeOffset Time { get; set; }
+
+    public int MeterId { get; set; }
+
+    public double Value { get; set; }
+
+    public int? SourceId { get; set; }
+
+    public ReadingQuality Quality { get; set; }
+
+    public ReadingFlags Flags { get; set; }
+
+    /// <summary>Provenance batch when this row came from a CSV/bulk import (revert support).</summary>
+    public int? ImportBatchId { get; set; }
+}