Add audiobook Date Added sorting and filtering

[therobbiedavis]

Summary

Supersedes #516 and moves the work onto a maintainer-owned branch so it can be completed and maintained without depending on write access to a contributor fork.

The original feature request and implementation originated with @T4g1 in #516 / discussion #513. This replacement preserves the existing #516 code state and attribution in the PR history while giving the Listenarr maintainers direct control of the branch for the remaining hardening and review work.

This feature adds a canonical Date Added value for audiobooks and exposes it throughout the library UI for sorting and custom filtering.

Changes

• Persist an audiobook's library-added timestamp.
• Expose added through library API/list DTOs.
• Add Date Added sorting to the audiobook library.
• Add Date Added as a custom-filter field with date comparisons.
• Preserve UTC semantics for persisted timestamps.
• Add legacy-upgrade/backfill support using durable library evidence where available.
• Add automated coverage for migration, serialization, sorting, and filtering behavior.

Supersedes

• Supersedes Allows sorting Audiobooks on added date #516.
• Original contribution: @T4g1.

Validation

The rebuilt #516 head was previously validated with the repository's backend/frontend CI and targeted Date Added regression coverage. Additional maintainer hardening identified during final review will be landed directly on this maintainer-owned branch before merge.

[therobbiedavis]

Remaining maintainer hardening from final #516 review

The maintainer-owned branch currently matches the last pushed #516 head (0859c661). Before marking this PR ready, carry over the already-reviewed local finishing commit 229f1984 (fix: finalize audiobook date-added contracts). That commit was validated locally but was never pushed to the contributor fork.

It contains these fixes:

• Remove the DateTime.UtcNow initializer from Audiobook.Added; assign the timestamp only at actual library-add commit boundaries.
• Preserve Added in detached AudiobookRepository.UpdateAsync so unrelated/partial metadata updates cannot rewrite lifecycle metadata.
• Make CollectionView Date Added sorting use the canonical added timestamp instead of audiobook ID.
• Make the built-in Recently Added filter use added rather than publication year while preserving its existing current/previous-calendar-year window.
• Expand backend/frontend regression coverage for UTC/null serialization, add-time persistence, detached-update preservation, CollectionView sorting, and Recently Added semantics.

Final validation performed on that exact local state:

• Release backend build: 0 warnings / 0 errors
• Backend: 2,999 passed, 156 platform-specific skips, 0 failed
• Frontend Vitest: 600/600 passed
• ESLint: passed
• Vue template-handler checks: passed
• Prettier: passed
• vue-tsc: passed
• Vite production build: passed
• Two independent adversarial review passes after the final changes: zero findings

Keep this PR in draft until that delta is landed on feature/date-added-sorting-filtering and CI reruns on the resulting GitHub head.