perf: cache library payload and batch the per-page *arr calls

[ndandan]

Summary

Speeds up the Radarr/Sonarr library pages (/medias/{slug}/films and /series) by caching the normalised library payload per instance and fetching the per-page *arr endpoints concurrently instead of sequentially.

Motivation

MediaController::films() / series() re-fetched and re-normalised the full getMovies() / getSeries() payload on every visit, and fetched status, queue, indexers, health and calendar one after another. So a navigate-away-and-back paid the full cost again, and a slow or unreachable instance stacked one timeout per call.

What changed

• MediaLibraryCache — a 45 s per-instance cache of the normalised library payload. Empty results are not cached (a transient failure isn't pinned for the window), and library mutations write-through-invalidate so user changes still show immediately.
• RadarrClient::multiGet() / SonarrClient::multiGet() — fetch the per-page endpoints in a single curl_multi batch instead of sequentially, with the same per-handle semantics as get(): SSRF protocol guard, CURLOPT_REDIR_PROTOCOLS, connect/total timeouts, and the per-instance circuit breaker.
• MediaController rewired to the cache + batch; the existing public normalizers are reused on the batch payloads, so there's no behaviour change to the rendered page.

Performance

In local testing (LAN, single run), the cold first load is unchanged, but a warm revisit within the cache window is ~3× faster (≈5.0 s → ≈1.4 s on my library). A slow/unreachable instance now costs one timeout window for the whole page instead of stacking one per call.

Tests

• MediaLibraryCacheTest — TTL, empty-not-cached, write-through invalidation.
• RadarrClientMultiGetTest — batch shape + fail-open semantics.
• MediaLibraryCacheInvalidationTest — mutations invalidate the entry.
• MediaLibraryPageTest — smoke test: the page renders cleanly when the *arr is unreachable (no 500, no hang).
• Existing MediaController tests updated for the new constructor dependency.

make check (lint + Twig + full PHPUnit) is green.

Definition of Done

• Works end-to-end; no leftover debug; English comments; no credentials; no unused imports
• Unit tests for the new logic; make check 100% green
• No schema/migration changes
• Read-only GETs; SSRF guard preserved on the batched calls; no leaked exception messages
• CHANGELOG [Unreleased] updated

Happy to split further, adjust naming, or tune the 45 s TTL / make it configurable if you'd prefer.