"Skip Secondary Series Books" never filters anything

[JordanFromIT]

Description

The Skip Secondary Series Books setting currently has no effect. Chaptarr writes every row in SeriesBookLink with IsPrimary = true, so no book is ever considered secondary and the filter has nothing left to remove — a series keeps every spin-off, companion volume, art book and anthology instead of just the main sequence. The metadata API already sends a per-book primary flag, and Chaptarr already carries it all the way to the domain model; the two places that write the link rows simply ignore it and hardcode true. This PR uses the value that already arrives. Two lines, plus a regression test.

For a concrete sense of the gap: Hardcover describes A Song of Ice and Fire as a 22-book series with 5 primary works. Entries like The Hedge Knight (#0.4) and The World of Ice & Fire (#0.5) sit at fractional positions precisely because they aren't part of the main sequence. With the option enabled today, all 22 are still kept.

The flag was already plumbed most of the way:

Step	Location	Before
API sends isPrimary per series slot	V5SeriesBook.IsPrimary — V5AuthorResponse.cs:423	✅ deserialized
Copied into the domain model	ConvertV5SeriesToDomain — BookInfoProxy.cs:2151	✅ IsPrimary = b.IsPrimary
Written to the link row	RefreshSeriesService.cs:160, :358	❌ hardcoded true
Read by the filter	MetadataProfileService.cs:238	✅ Any(y => y.IsPrimary)

Both write sites now read seriesBook.IsPrimary ?? true. The ?? true matters: SeriesBook.IsPrimary is nullable, and its own comment says consumers should treat null as primary so that authors who haven't been refreshed since the field was added don't silently lose series members. That's also exactly what BookInfoProxy.cs:1171 already does — this just brings the other two sites in line with the existing precedent rather than inventing a convention.

Blast radius is small. SeriesBookLink.IsPrimary has exactly one functional consumer, the SkipSeriesSecondary filter itself. Every other IsPrimary in the tree belongs to a different entity (BookNarratorLink, EditionNarratorLink, NarratorCredit), and SeriesVariantService, RefreshEntityCopy and LocalEdition only copy the value through. So flipping a previously always-true flag to sometimes-false doesn't change behaviour anywhere except the option that was supposed to depend on it.

Fixes #60

Database Migration

NO. The IsPrimary column already exists on SeriesBookLink (001_chaptarr_complete_schema.cs:435, default true). Only the value written into it changes. Existing rows are corrected on the next author refresh; nothing needs backfilling, and unflagged books keep their current primary status by design.

How was this tested?

Built and tested on Linux, .NET 10, from source — not through Docker. Ran the same steps build.yml uses, including the guards that run before compilation:

• git grep conflict-marker check + package.json JSON parse — clean.
• python3 build-tools/version_guard.py sync — Version sync OK for working tree: 0.9.929.
• python3 build-tools/version_guard.py monotonic --compare-ref origin/develop — Version monotonic OK: 0.9.929.
• python3 build-tools/version_guard.py commit-hygiene --compare-ref origin/develop — OK for range origin/develop..HEAD.
• dotnet build src/Chaptarr.NoTests.sln --configuration Release — succeeded, 0 warnings, 0 errors (so TreatWarningsAsErrors / EnforceCodeStyleInBuild are satisfied).
• dotnet test src/Chaptarr.Core.Test/Chaptarr.Core.Test.csproj --configuration Release — 2834 on develop → 2835 on this branch, 0 failed. The one added test is the delta.
• Added should_carry_series_book_primary_flag_into_links to RefreshSeriesServiceExistingSeriesFixture, covering all three states through the path that actually persists links: isPrimary = true → linked primary, isPrimary = false → linked secondary, isPrimary = null → linked primary (the back-compat case).
• Confirmed the test genuinely covers the bug by reverting the fix and re-running it: it fails with a book the metadata API flags as secondary should be linked as secondary / Expected: False / But was: True, then passes again with the fix restored.

I have not exercised this end-to-end against a live library — the change is two lines with a single consumer, so the unit tests seemed a better proof than a manual refresh. Happy to run a real-instance check if you'd like one before merging.

Screenshots (UI changes only)

n/a — no UI changes.

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Two adjacent things I deliberately left out of this PR, mentioned in #60 in case they're worth their own tickets: WorkCount and PrimaryWorkCount are assigned the same value at BookInfoProxy.cs:4661-4662 and SeriesLookupController.cs:222-223, so the ({count} Primary) label can never differ from the total; and Series.TotalBooks / Series.PrimaryBooks are written by some converters but not others, leaving them zero on rows that took the other path. Neither affects filtering, and the first looks like it may be deliberate for search previews, so I didn't want to guess at intent inside a bugfix.