use browser headers for audiobookbay

[sgerner]

What changed

• AudioBookBay requests now use a browser-like header set instead of the generic Librarr UA.
• The ABB search path and ABB magnet resolver both apply the same headers.
• Added tests that assert the outbound ABB request sends the expected User-Agent and related browser headers.

Why

AudioBookBay was rejecting librarr’s default request identity in this environment. A browser-style request profile is required for the ABB search and detail pages to return usable HTML.

Impact

• ABB search results should continue to work without a proxy workaround.
• Magnet resolution for ABB detail pages uses the same request profile.
• Other search sources are unchanged.

Validation

• go test ./internal/search -run 'TestAudioBookBay|TestResolveABBMagnet|TestRegistryFlow'
• go test ./internal/search

Notes

This fix is intentionally scoped to AudioBookBay so it does not change the request identity used by the rest of librarr.