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Jellyfin libraries typed `mixed` were dropped by JellyfinApiClient.Project's `_ => None` with no log line, so music and standup content could not be ingested without a local-library workaround that bypassed Jellyfin entirely. Adds LibraryMediaKind.Mixed, maps "mixed"/absent/blank CollectionType onto it, and gives SynchronizeJellyfinLibraryByIdHandler a Mixed arm composing the three existing per-kind scanners. Jellyfin classifies items server-side via includeItemTypes, so the passes see disjoint sets; reconciliation is type-scoped and cannot cross-delete. No new scanner and no DB migration -- MediaItem is TPT keyed on LibraryPathId, so heterogeneous contents were already legal. Segregation falls out of the model: a library is a place (one path <-> one Jellyfin library <-> one ErsatzTV library), so music/standup cannot leak into Movies or TV Shows. Also removes the silent-success `_ => Unit.Default` from both scanner dispatchers, which returned Right for an unhandled kind and stamped LastScan as though a scan had run, and rejects Mixed for local libraries at the API. Deliberately Jellyfin-only: local scanners share one video extension list and would claim each other's files, and LibraryFolder etags are keyed by LibraryPathId with no notion of kind. Verified by live E2E against a real Jellyfin, including the interaction with #494's reconciliation sweep. Four cold review rounds, all MERGEABLE. fixes #489 Co-authored-by: Timothy <timothy.look@gmail.com> Co-committed-by: Timothy <timothy.look@gmail.com>