Fix #28: author audiobook/ebook path edits are silently dropped

[sebclark]

Fixes #28

The bug

Editing an author's Audiobook or Ebook path in the UI never saves — the request succeeds, nothing is logged, and the value reverts. The reporter's devtools captures show why: the frontend submits audiobookPath/ebookPath, but AuthorResource has no such properties (the API's only per-media path fields are the read-only audiobookFolder/ebookFolder display values), so the deserializer silently drops the incoming value. Not related to spaces in author names — no author path edit can save.

The value was actually being dropped at two layers:

1. AuthorResource → model: the fields don't exist on the resource, and ToModel never maps them
2. Author.ApplyChanges (the final merge for plain updates): copies Path and the root-folder paths but not AudiobookPath/EbookPath

Fix

• Add writable AudiobookPath/EbookPath to AuthorResource (the read-only folder display fields are unchanged), map them in both directions
• Preserve-when-null in the facade update path, mirroring the existing Path handling
• Merge them in Author.ApplyChanges with ?? semantics: null means not-provided (keep stored), an explicit clear arrives as an empty string
• PutValidator path validation for both, only when set

Tests

AuthorResourceMapperFixture, following the fixture's existing apply/not-wipe pattern:

• should_apply_media_paths_on_put_update — provided value lands; omitted sibling keeps its stored value
• should_not_wipe_media_paths_when_not_provided_on_put_update

Mapper suite 102/102; wider Books suite 567/567.