feat(489): support Jellyfin mixed-content libraries (#493)

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>

web/src/api/generated/v1.d.ts

@@ -813,7 +813,7 @@ export interface components {
     "seasonId"?: null | number;
   };
     "LibraryBrowseMediaType": "Movie" | "TelevisionShow" | "TelevisionSeason" | "Artist" | "Collection" | "SmartCollection" | "MultiCollection" | "RerunCollection" | "Playlist" | "Episode" | "MusicVideo" | "Song" | "OtherVideo" | "Image" | "RemoteStream";
-    "LibraryMediaKind": "Movies" | "Shows" | "MusicVideos" | "OtherVideos" | "Songs" | "Images" | "RemoteStreams";
+    "LibraryMediaKind": "Movies" | "Shows" | "MusicVideos" | "OtherVideos" | "Songs" | "Images" | "RemoteStreams" | "Mixed";
     "LibraryScanStatusResponseModel": {
     "libraryId": number;
     "percent": number;