feat: show audiobooks under all their series and fix the Primary-series toggle (#658)

[s3ntin3l8]

Summary

The backend already models books that belong to multiple series (AudiobookSeriesMembership join with IsPrimary/SortOrder, populated by Audible/Audnexus), but the UI and one list query still treated each book as single-series. As a result a book that's in two or more series only showed under its provider-chosen primary, and the per-book "Primary" series toggle silently reverted on save.

This PR makes a book appear under every series it belongs to and makes the Primary selection actually stick — which also makes {Series} folder naming follow the chosen series.

Motivating example: a series catalogued in both publication order and chronological order (e.g. Jack Ryan) previously split across two half-empty series with no way to pick the canonical one.

Closes #658.

Changes

Added

• Books appear under all their series: the library "Series" grouping and the series collection view list a book under each of its series memberships (not just the primary), showing the correct position number for that series. Book cards and detail modals list every series the book belongs to.
• Series memberships in the library list payload: LibraryAudiobookListItem now carries SeriesMemberships, batch-loaded in LibraryListService via a new GetAllSeriesMembershipsGroupedByAudiobookIdAsync repository query (mirrors the existing file-summary batching — no per-row Include, no change to the shared GetAllAsync).

Fixed

• "Primary" series toggle now persists: the edit dialog's save payload built isPrimary: Boolean(membership.isPrimary || index === 0), which re-flagged the first series as primary alongside the user's pick; the backend keeps the first primary it finds, so the choice reverted to the provider default. The payload now sends only the user's selection.
• Metadata rescan keeps the chosen primary: a manual "Rescan Metadata" replaced memberships with provider data and re-derived primary as the provider's first. A new AudiobookSeriesMembershipHelper.ApplyToAudiobookPreservingPrimary re-applies the user's chosen primary when the provider still returns that series (falling back to the provider default otherwise).
• Latent DbContext concurrency hardening (independent of the feature): LibraryListService started the file-summary, file-count, and series-membership reads concurrently on the shared scoped ListenArrDbContext; under real database latency that risks EF Core's "a second operation was started on this context instance" error (SQLite masks it today by running the async path synchronously). Those same-context reads now run sequentially, while the download read — which uses its own IDbContextFactory context — stays parallel.

Testing

• Backend dotnet test: full suite green, including new unit tests for ApplyToAudiobookPreservingPrimary and the Normalize non-first-primary case, a library-list payload test asserting seriesMemberships, and a RenameService regression asserting {Series} folders under the chosen primary.
• Frontend vitest: full suite green, including a new EditAudiobookModal test asserting the save payload keeps a non-first primary (index 0 = isPrimary:false) and seriesUtils unit tests.
• vue-tsc type-check, ESLint, Vue template-handler check, Prettier, and dotnet format all clean.

Notes

• No DB migration — the AudiobookSeriesMemberships table already exists; this is display + persistence wiring only.
• Related PR [Feature Request] Sort Series by the order of the book in the series #626 ("sort a series by reading order") is independent and also based on canary — the two compose (this PR gives [Feature Request] Sort Series by the order of the book in the series #626 correct per-series numbers for multi-series books) but neither depends on the other, and they can merge in any order.
• Overlap with Implicit patterns added to configured naming patterns #571 (PR fix: apply naming patterns exactly and unify all naming flows (#571) #657): this PR's new RenameServiceTests test is placed at the end of the class specifically so it no longer collides with Implicit patterns added to configured naming patterns #571's additions — the two now auto-merge in RenameServiceTests.cs regardless of merge order.
• Needs a version label.

[therobbiedavis]

Thanks, this is a nicely focused fix and the regression coverage is good. I left two comments: one backend concurrency risk around starting multiple EF queries on the same scoped DbContext, and one UI gap where the series collection view resolves the per-series number but never renders it. Also the changelog is no longer needed.

[s3ntin3l8]

Fixed the points from the review, also pushed two follow-ups that I noticed while testing:

• "Not Added" items now show the viewed series' position, not the book's primary-series number. The series catalog was mapping each book via book.Series.FirstOrDefault() (its primary); it now selects the entry for the series being viewed, so a book that's Update workflows: add Docker login and fix indent #3 in this series no longer shows its primary's Bump version to 1.0.1 #5. Library items were already correct. (Covered by a new test.)

• Renamed GetSeriesMembershipsByAudiobookIdsAsync → GetAllSeriesMembershipsGroupedByAudiobookIdAsync — it takes no IDs and returns the whole AudiobookSeriesMemberships table grouped by audiobook id, so the name now matches the behavior (no more "expects IDs" footgun). Pure rename, no behavior change.

I also called out the LibraryListService sequential-await change in the description as the latent DbContext "second operation" fix, per the review note.

These are separate commits for easier review — can be squashed before merge.

[therobbiedavis]

This one LGTM as well!