fix(datacopy): tag copied timestamps as UTC instead of enabling legacy behaviour

DataCopy read SqlServer datetime2 values, which carry Kind=Unspecified, straight into Postgres timestamptz columns that reject them. It previously worked around this with the process-wide Npgsql legacy switch; now every copied entity has its DateTime and DateTime? properties normalized to Kind=Utc before the write. That names the existing value without shifting it, so the stored instant stays exactly what SqlServer held, and no global behaviour is changed.

src/GreenHome.DataCopy/Program.cs

@@ -47,12 +47,6 @@ internal static class Program
             ?? throw new InvalidOperationException("Missing ConnectionStrings:Postgres");
         var reset = args.Contains("--reset");
 
-        // ponytail: this tool passes DateTime values through verbatim (SqlServer datetime2 reads
-        // back as Kind=Unspecified, the API's timestamptz columns reject that). Legacy behaviour
-        // makes Npgsql accept either Kind for this pass-through copy only — the API process does
-        // not enable it. Retire this if the copy ever has to reinterpret time zones.
-        AppContext.SetSwitch("Npgsql.EnableLegacyTimestampBehavior", true);
-
         using var source = new GreenHomeDbContext(
             new DbContextOptionsBuilder<GreenHomeDbContext>().UseSqlServer(sqlServerCs).Options);
         // the target must be the Postgres-specific context so Migrate() picks the Postgres
@@ -170,6 +164,7 @@ internal static class Program
                 .Where(where).OrderBy(orderBy).Take(batchSize).ToListAsync();
             if (batch.Count == 0) break;
 
+            foreach (var item in batch) NormalizeTimestampsToUtc(item);
             target.Set<TEntity>().AddRange(batch);
             await target.SaveChangesAsync();
             copied += batch.Count;
@@ -204,6 +199,7 @@ internal static class Program
         foreach (var row in await source.Set<TEntity>().AsNoTracking().ToListAsync())
         {
             if (existing.Contains(KeyOf(row, pkNames))) continue;
+            NormalizeTimestampsToUtc(row);
             target.Set<TEntity>().Add(row);
             existing.Add(KeyOf(row, pkNames));
             copied++;
@@ -217,6 +213,32 @@ internal static class Program
     // Plumbing
     // ============================================================
 
+    /// <summary>
+    /// SqlServer's datetime2 carries no Kind, so values are read back as Unspecified. The app has
+    /// always written UTC (DateTime.UtcNow) and the target columns are timestamptz, which rejects an
+    /// Unspecified value. Tag every timestamp as Utc before it is written — this names the value,
+    /// it never shifts it, so the stored instant stays exactly what SqlServer held.
+    /// </summary>
+    static void NormalizeTimestampsToUtc(object entity)
+    {
+        foreach (var property in entity.GetType().GetProperties())
+        {
+            if (!property.CanRead || !property.CanWrite) continue;
+
+            if (property.PropertyType == typeof(DateTime))
+            {
+                var value = (DateTime)property.GetValue(entity)!;
+                if (value.Kind != DateTimeKind.Utc)
+                    property.SetValue(entity, DateTime.SpecifyKind(value, DateTimeKind.Utc));
+            }
+            else if (property.PropertyType == typeof(DateTime?))
+            {
+                if (property.GetValue(entity) is DateTime value && value.Kind != DateTimeKind.Utc)
+                    property.SetValue(entity, DateTime.SpecifyKind(value, DateTimeKind.Utc));
+            }
+        }
+    }
+
     static async Task<object?> ScalarAsync(DbConnection conn, string sql)
     {
         await using var cmd = conn.CreateCommand();