Bulk import enhancement: group tagless files by folder and use folder name as search fallback

[MattiasC]

Previously, audio files with no ID3 metadata tags were each treated as a
separate audiobook entry, producing N results for a single folder instead
of one. The search term also fell back to the individual file name (e.g.
"The Gunslinger 01-05"), which rarely matched correctly on Audible.

• Group all no-metadata files in the same folder into one entry instead
  of one entry per file
• Use folder name as search term fallback before file name (skipping
  generic names like CD1, Disc 2, Part 3)
• Extract Audible ASIN directly from folder names (e.g. "(B019NOKST6)")
  and use a direct ASIN lookup, skipping the text search entirely
• Suppress "Low Confidence" badge when match came from an ASIN lookup
• Pass extractedAsin through SSE events and into the ScannedBook type

Claude has been used when creating this PR.

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Test of a folder containing an ASIN in the name:
rmab_import_log.txt

Test of a folder without ASIN in the name:
rmab_import_log_no_asin.txt