fix(search): fall back to default quality profile when audiobook has none, and expose bulk-assign in Wanted view

[kevinheneveld]

Summary

Drafted for visibility per the pacing note on #590 — happy to leave this in draft until you have time.

Two related symptoms users hit when an audiobook is monitored but doesn't have a quality profile assigned:

1. Automatic search silently returns 0: AutomaticSearchService.ProcessAudiobookAsync and DownloadService.SearchAndDownloadAsync both early-return when audiobook.QualityProfile == null. An unconfigured book is monitored, shows up in Wanted, but never gets searched — the user has no feedback as to why.
2. No way to fix it in bulk: Setting a profile requires opening every wanted book's edit modal. With a default profile configured in settings, the user expects it to apply.

Fix

Backend: Both auto-search paths now fall back to qualityProfileService.GetDefaultAsync() when the audiobook has no profile assigned. If a default is configured the audiobook gets searched as if that were its profile (in-memory only — no DB write). If there's no default either, log the reason and return 0 as before.

Frontend: A new "Assign default profile (N)" button on the Wanted view appears only when at least one wanted book has no profile and a default is configured. Clicking it bulk-assigns the default to every such book via the existing PUT /library/{id} endpoint. Per-book failures are surfaced individually but don't stop the batch.

Also: the Wanted row's quality column used to show item.quality ?? 'Unknown' for profile-less books; now shows "No profile" so the cause is visible at a glance.

Test plan

• Backend builds clean
• Frontend builds clean (npm run build)
• Verified in a downstream deploy: a previously-stuck wanted book gets searched as soon as a default profile is configured (no longer requires per-book edit + save round-trip).

🤖 Generated with Claude Code

[therobbiedavis]

Hey @kevinheneveld Would you mind joining us on https://discord.gg/CwZ2Sqp9NF

We're working on prioritizing PRs and you've opened a good number of them, so I figured you should be involved. 😜