fix(playback): wait for mpv's reply to every reused-instance IPC command (#1876)

Every reused-instance MPV IPC command now carries a request_id and waits, within the existing 2 s bound, for mpv's matching reply; an error or missing reply falls back to a fresh launch like a socket failure, and the racing post-loadfile seek is gone. MPV launches run one at a time through a launch queue so overlapping launches never act on the same child; Stop on a queued launch delegates to the in-flight launch's own closer, a failed exit stays retryable, and the dock keeps the newest launch. Position polling and output-error reporting moved out of mpv-session.service.ts, which leaves the max-lines baseline.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/eslint/max-lines-baseline.mjs

@@ -13,7 +13,6 @@ export const maxLinesBaseline = [
     'apps/electron-backend/src/app/database/operations/content.operations.ts',
     'apps/electron-backend/src/app/database/operations/playlist.operations.ts',
     'apps/electron-backend/src/app/events/epg-query.service.ts',
-    'apps/electron-backend/src/app/events/mpv-session.service.ts',
     'apps/electron-backend/src/app/events/vlc-session.service.ts',
     'apps/electron-backend/src/app/services/app-update.service.ts',
     'apps/electron-backend/src/app/services/embedded-mpv-native.service.ts',