Final review follow-up for ABS Phase 0. Five fixes:
1. loginEnvelope and handlePersonalized now nil-guard h.deps.MediaStore
so a partially-wired deployment doesnt panic on /login or /personalized.
2. handleLibraryAuthors and handleLibrarySeries respect ABS limit=0
("return all") instead of returning an empty slice.
3. buildSiloAudioTracks now sets MetaTags to map[string]string{} so the
"spinner forever" failure mode types.go warns about cannot bite the
play-session response (item-detail path already sets it; play path
silently omitted the key).
4. slog key normalised from "error" to "err" in the play-session
persist-failure log so log parsers find it.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
