fix(metadata): populate SeriesAsin from the Audnexus series record

[m4bard]

Summary

Fixes #767.

AudnexusSeries carries three fields:

public class AudnexusSeries
{
    public string? Asin { get; set; }
    public string? Name { get; set; }
    public string? Position { get; set; }
}

ConvertAudnexusToMetadata mapped Name and Position but dropped Asin, so AudiobookSeriesMembership.SeriesAsin was null for every audiobook — even though the column exists for it (AddAudiobookSeriesMemberships, maxLength: 64) and the repository preserves it when it is set.

Why the series ASIN matters: it is the stable identifier for a series, and neither of the alternatives is.

• A book ASIN is per-marketplace and per-narrator. Grimm's Kinder- und Hausmärchen is B00B4FPO6A in the German catalogue and B00TPKFANI in the US one, and each returns 404 from the other region. Re-releases mint new book ASINs as well.
• A series name is free text and varies between editions and translations.
• The series ASIN is stable across both.

So (SeriesAsin, SeriesNumber) is the durable way to express "book N of that series", and it was unavailable because the ASIN was dropped at conversion.

Changes

Fixed

• MetadataConverters.ConvertAudnexusToMetadata now copies AudnexusSeries.Asin into AudiobookSeriesMembership.SeriesAsin, for both the primary and the secondary membership.

Testing

MetadataConvertersSeriesAsinTests — 3 new cases, built from the live Audnexus record for H. Rider Haggard's She and Allan (B00CQ5WAXW), which belongs to two series at once:

"seriesPrimary":   { "asin": "B01E633FQM", "name": "Ayesha",           "position": "0" },
"seriesSecondary": { "asin": "B01F5TL5K4", "name": "Allan Quatermain", "position": "7" }

• the primary membership carries its series ASIN
• the secondary membership carries its series ASIN
• a book in two series keeps both memberships (this already worked; pinned so it stays working)

dotnet test: 1192 passing, 0 failing (1189 existing + 3 new). No existing test changed.

Notes

Two lines. The column, the DTO field, the dual-membership handling and the persistence merge were all already in place — only the mapping was missing.

The book cited is public domain with a real Audible edition, so the case is reproducible by anyone.