fix(#297): reconcile FeedDB category membership on each fetch

[cassio-rossi]

Summary

• FeedDB.categories was updated with a pure union on every fetch (existing.categories = Array(Set(existing.categories + feed.categories))), so a synthetic category key (e.g. NewsCategoryHighlights) could only ever be added, never removed — a post de-highlighted (or otherwise re-categorized) on the server kept showing under its old category forever.
• getFeed(page:) now fetches all six categories (5 special + news) in parallel and hands each category's result to a new grouped save that reconciles membership within the pubDate window that fetch actually returned: it adds the category key to matches as before, and removes it from any locally stored post inside that window which the fetch no longer includes.
• Reconciliation is scoped per category and by date window (not page number), so multi-category posts and deep pagination (different categories paginate at different rates) are unaffected — a fetch only ever touches its own key, within its own window. .news is excluded from removal since every post belongs to it.

Test plan

• xcodebuild build — BUILD SUCCEEDED (iPhone 17 Pro simulator unavailable on the build machine; substituted iPhone 17)
• xcodebuild test -testPlan MacMagazine — 523 checks passed, 0 failures
• New StorageServiceTests cases: add via grouped save, stale-key removal, sibling-category preserved, empty-result no-wipe, out-of-window safety, .news excluded from reconciliation
• swiftlint lint --strict — 0 violations on changed files

Known limitation (documented in code review, not fixed here): if a highlight is rotated out and happens to be the single newest/oldest post in its batch, its pubDate can fall just outside the remaining window and survive one extra refresh cycle before clearing. Acceptable for the curated-highlights case.

Co-Authored-By: Claude noreply@anthropic.com