fix(import): finalize pending imports without file-filter gate; fail closed on Arr queue errors

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Summary

Fixes two regressions introduced in v6.12.4 (f49384b) when import completion was deferred until Arr queue confirmation.

1. Finalize stall for complete-first torrents — TryFinalizeImportedTorrentAsync only ran inside Branch 14, which still required IsFileFiltered. Torrents first seen in a complete/seeding state (cross-seeds, manual adds) skip Branch 9 file filtering, so Arr-confirmed imports could stall forever with Imported=false, tags never applied, and AutoDelete never firing.
2. Fail-open on Arr API errors — IsImportedAsync treated queue polling failures as "not in queue" and returned imported=true, which could prematurely mark DB rows and trigger AutoDelete while the item was still queued (e.g. during Arr restarts).

Testing

• Added or updated automated tests
• Performed manual verification (describe below)
• Not applicable (explain why)

Manual test notes:

N/A — covered by unit tests.

New tests:

• ProcessSingleTorrentAsync_PendingImport_FinalizesWithoutFileFilterWhenArrConfirms
• IsImportedAsync_QueueCheckFails_ReturnsFalse

dotnet test --filter "Category!=Live" — 781/781 pass (173 Core + 405 Infrastructure + 203 Host).

Checklist

• Linked the related issue or discussed scope with maintainers
• Ran dotnet format and npm run lint (if touching frontend)
• Updated documentation or release notes where it makes sense
• Confirmed there are no sensitive secrets or credentials committed

Additional Notes

Automated bug-finding scan for commits e94fab5..949198b. Deduplication: searched open PRs (#322, #329 dep-only), merged PRs (14 days), and open issues (0) — no existing fix found.