[bilipp]

Problem

iCloud sync caused heavy UI lag — even on the latest hardware the app felt completely stuck during sync, and sync seemed to fire at random / too often (on every foregrounding, on opening a movie detail, etc.).

Root cause

The work itself was already off-main, but two things hit the main thread:

1. Redundant reconcile passes. Foregrounding fired a full reconcile every time; each reconcile that produced changes auto-merged into the main context, re-running every on-screen @Query. The "opening a movie detail" lag was actually TMDB/OMDB enrichment saving synchronously on the view's main context (scalar fields + a full cast replace) on every detail open.
2. An unconditional full re-encode of the entire sync shadow baseline to UserDefaults JSON on every pass, even when nothing changed.

(At the transport layer CloudKit already syncs deltas; the reconcile scan is bounded by user-state size, not catalog size — so an incremental/persistent-history rewrite was deliberately not pursued: it would be dominated by catalog-sync churn and add fragility for no real gain.)

Changes

• Gate foreground reconciles behind a 30s interval (CloudSyncCoordinator.minForegroundReconcileInterval). Quick app-switches no longer re-merge for no new data; genuine remote changes still pull immediately via the remote-change observer.
• Move TMDB + OMDB enrichment off the main thread. ContentSyncManager now owns enrichMovie / enrichSeries / enrichMovieRatings / enrichSeriesRatings, which fetch + apply + save on a background context (auto-merging into the main context). All 5 detail views + Home hero-logo enrichment call these instead of applying on modelContext + a synchronous save. applyMovieDetails/applySeriesDetails/replaceCast and the TMDBTitleDetails/TMDBCastMember DTOs are now nonisolated.
• Skip the shadow re-encode when unchanged. CloudSyncShadow tracks a dirty flag; persist() no-ops in steady state.

Testing

• Builds clean (iOS 26.5 sim).
• CloudSyncTests and ContentSyncTests pass (the one failing ContentSyncTests case is a pre-existing bulk-insert benchmark, unrelated — it touches none of this code).
• swiftformat 0.61.1 + swiftlint 0.63.2 clean.

Note for review

This reverses the original code's deliberate "apply enrichment on the view context to avoid cross-context merge timing issues" choice. The awaited background save + auto-merge, followed by refreshToken = UUID(), is the safety net. Episodes still load on the view context on purpose (instant relationship display) and are unchanged. Worth a sanity check that enriched text/cast/logos still appear promptly on detail open.