separate Prowlarr audiobook results from ABB

[sgerner]

What changed

• Prowlarr audiobook search results now use the distinct source label prowlarr_audiobooks instead of audiobook.
• Updated the corresponding unit test to assert the new label.

Why

Librarr already treats audiobook as the ABB source. That causes Prowlarr audiobook NZB results to be filtered as if they were ABB rows, which drops them when they do not have ABB-specific metadata or seeders.

Impact

• Prowlarr audiobook results from indexers such as nzb.life now survive librarr's filter path.
• ABB search behavior is unchanged.
• The change is scoped to librarr result labeling only; no Prowlarr configuration changes are required.

Validation

• go test ./internal/search -run 'TestProwlarr|TestIsNZBURL'
• Built a temporary image from the patched source and ran a smoke test against GET /api/search/audiobooks?q=The Martian.
• Confirmed the returned results include source: "prowlarr_audiobooks" and indexer: "Nzb.life".

Retracting — I was wrong. GetSource catches prowlarr_audiobooks via Prowlarr's Name() so the download.go and requests.go literal checks are unreachable for this label. filter.go:155 is a flat-2 priority instead of 1-if-seeded but that only matters for seeded results, not NZBs (this PR's actual target).

[JeremiahM37]

One more spot to update: internal/search/scorer.go:267 — add "prowlarr_audiobooks" to the audiobook case in guessMediaType, otherwise size scoring falls through to the ebook range and a typical audiobook NZB drops from 10 to 3.

[JeremiahM37]

Related: internal/search/scorer.go:267 (guessMediaType) still only maps Source=="audiobook" to the audiobook size band. Prowlarr doesn't set MediaType on its results, so prowlarr_audiobooks now falls through to ebook here and a 500MB-2GB audiobook NZB drops from a size score of 10 to 3. Add "prowlarr_audiobooks" to that case too.