Phase E: Add WHATS_NEW entry for media server engine refactor

Internal-track entry covering the media server engine + contract +
the honest-scope note explaining why we lifted the 4 truly-uniform
is_connected dispatches and left the deep server-specific dispatches
explicit (each does fundamentally different work per server, so
lifting would just move per-server branches into engine helper
methods).

webui/static/helper.js

@@ -3432,6 +3432,7 @@ const WHATS_NEW = {
     '2.4.2': [
         // --- post-2.4.1 dev work — entries hidden by _getLatestWhatsNewVersion until the build version bumps ---
         { date: 'Unreleased — 2.4.2 dev cycle' },
+        { title: 'Internal: Media Server Engine Foundation', desc: 'internal — companion to the download engine refactor. introduces a media-server engine + plugin contract on top of the four server clients (plex / jellyfin / navidrome / soulsync standalone). web_server.py historically had ~33 `if active_server == "plex" / "jellyfin" / ...` dispatch sites. new `core/media_server/` package provides `MediaServerEngine` that reads `server.active` config + routes to the right client. plugin contract narrowly requires only the four methods every server actually implements (is_connected, ensure_connection, get_all_artists, get_all_album_ids); optional methods (search_tracks, trigger_library_scan, get_library_stats, etc.) are routed with safe defaults so SoulSync standalone (no library scan API since it walks the filesystem directly) doesn\'t need stub methods. lifted the four uniform `is_connected` dispatches into `engine.is_connected()`. honest scope: most "dispatch sites" the recon counted are genuinely different per server (playlist track replace, per-server metadata sync, deep scan with server-specific cache strategies) — those stay explicit per the "lift what\'s truly shared" standard. 35 new tests pin: per-server observable behavior (4 server pinning files, 21 tests), engine cross-server dispatch (10 tests), structural conformance (4 tests). engine reference + plugin contract available for future targeted refactors. zero behavior change for users.' },
         { title: 'Internal: Typed Metadata Foundation', desc: 'internal — first step of a multi-pr migration to give the metadata pipeline a real contract. the codebase historically grew duck-typed extractors (`_extract_lookup_value(album_data, "id", "album_id", "collectionId", "release_id", default=...)`) at every consumer site because each provider returns its own response shape. ~150 of those across the codebase. new `core/metadata/types.py` defines canonical typed `Album` / `Track` / `Artist` dataclasses with strict required fields. per-source classmethod converters (from_spotify_dict, from_itunes_dict, from_deezer_dict, from_discogs_dict, from_musicbrainz_dict, from_hydrabase_dict) are the SINGLE place that knows each provider\'s wire shape. zero behavior changes in this pr — pure additive foundation. follow-up prs migrate consumers one at a time. full migration plan documented at docs/metadata-types-migration.md.', page: 'library' },
         { title: 'Internal: Migrate Album-Info Builders to Typed Path', desc: 'internal — steps 2+3 of the typed metadata migration in one pr. two album-info builders now route through `Album.from_<source>_dict()` when the caller passes a known source: `_build_album_info` (used by every album-tracks lookup) and the embedded album section of `_build_single_import_context_payload` (used by single-track import context resolution). legacy duck-typed extraction stays as the fallback when source is empty/unknown, raw input isn\'t a dict, or the typed converter raises — so a converter bug can\'t break album resolution or import context. caller-provided album_id / album_name / artist_name fallbacks apply on the typed path the same way they did on legacy. zero behavior change for existing callers since they don\'t pass a source yet — opt-in only. 22 new tests pin the typed path, the legacy fallback, and parametrized coverage across registered providers.' },
         { title: 'Internal: Migrate Discography + Quality Scanner to Typed Path', desc: 'internal — next round of the typed metadata migration. three more album-shape consumers now route through `Album.from_<source>_dict()` when the caller passes a known source: `_build_discography_release_dict` (artist discography release cards), `_build_artist_detail_release_card` (artist detail page release cards), and `_normalize_track_album` (quality scanner result normalization). legacy duck-typed extraction stays as the fallback when source is empty/unknown, raw input isn\'t a dict, or the typed converter raises — same safety contract as the prior migration steps. 20 new tests pin the typed path + legacy fallback + parametrized coverage across registered providers.' },