Import: reject duplicate reading targets in the CSV wizard

Mapping two Reading columns onto one meter staged two readings per row at the
same timestamp. (meter_id, time) is the reading key, so the commit failed with
a raw EF change-tracker error that named neither the column nor the meter.

- Wizard Validate() rejects duplicate Reading targets, naming the columns and
  the meter. Reading-only: Delivery/TankLevel stage events, not readings.
- CommitAsync() re-validates. Previously only Preview did, so a mapping edited
  after a dry run reached the database unchecked.
- ImportService.GuardDuplicateReadings() backstops the same case for the API
  and any other non-wizard caller, reporting meter + date.

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

src/App/Components/Pages/ImportWizard.razor

@@ -292,6 +292,14 @@
             return;
         }
 
+        // The mapping can change after a dry run — re-check before writing.
+        _validationErrors = Validate();
+        if (_validationErrors.Count > 0)
+        {
+            _staged = null;
+            return;
+        }
+
         _committing = true;
         try
         {
@@ -341,6 +349,21 @@
             }
         }
 
+        // Two Reading columns on one meter would stage two readings per row at the same timestamp,
+        // and (meter_id, time) is the reading key — reject it here rather than at the DB.
+        var duplicateTargets = _columns
+            .Select((c, i) => (Column: c, Index: i))
+            .Where(x => x.Column.Role == MappingRole.Reading && x.Column.MeterId is not null)
+            .GroupBy(x => x.Column.MeterId!.Value)
+            .Where(g => g.Count() > 1);
+
+        foreach (var group in duplicateTargets)
+        {
+            var meterName = _meters.FirstOrDefault(m => m.Id == group.Key)?.Name ?? $"meter {group.Key}";
+            var cols = string.Join(", ", group.Select(x => $"Col {x.Index}"));
+            errors.Add($"{cols} all read into '{meterName}'. Each Reading column needs its own meter.");
+        }
+
         return errors;
     }