feat(plex): transcode resource discipline + admin knobs (E6 S1c)

The software-fallback re-encode now runs deprioritized and admin-configurable.
The SW transcode ffmpeg child is niced (preexec_fn → os.nice, applied per-child
because the Popen lives in the long-lived API process, which must NOT itself be
niced — unlike the download worker's whole-process _deprioritize) so a CPU-bound
re-encode can't starve request handling or playback; the VAAPI path needs none of
it.

New sysconfig knobs (plex group, rendered on the admin Configuration page, i18n
HU/EN): plex_transcode_hw (auto/on/off, layered onto _want_hw), plex_transcode_
preset, plex_transcode_crf, plex_transcode_max_height (software-only downscale-to
cap; the GPU path encodes at source resolution). plex_max_transcodes stays the
concurrency cap (comment corrected — the fallback is shipped, not "later").

pytest extends the command-builder + _want_hw coverage (preset/crf/max_height
plumbing, the on/off/auto policy). Browser-UAT'd: max_height=240 drops the live
resolution to 330x240; the SW ffmpeg child runs at nice 10 (ppid=api process).

backend/app/sysconfig.py

@@ -159,6 +159,14 @@ SPECS: tuple[ConfigSpec, ...] = (
     ConfigSpec("plex_overlay_map", "str", "plex", "plex_overlay_map"),
     # (plex_sync_interval_min is tuned from the Scheduler dashboard, not here — it's a scheduler job.)
     ConfigSpec("plex_max_transcodes", "int", "plex", "plex_max_transcodes", min=0, max=16),
+    # Transcode re-encode tuning (E6 S1). hw = "auto"|"on"|"off" (free-text, like search_source); the
+    # libx264 knobs apply only to the software fallback. crf 0–51 (x264 range); max_height 0 = source.
+    ConfigSpec("plex_transcode_hw", "str", "plex", "plex_transcode_hw"),
+    ConfigSpec("plex_transcode_preset", "str", "plex", "plex_transcode_preset"),
+    ConfigSpec("plex_transcode_crf", "int", "plex", "plex_transcode_crf", min=0, max=51),
+    ConfigSpec(
+        "plex_transcode_max_height", "int", "plex", "plex_transcode_max_height", min=0, max=4320
+    ),
     # NOTE: plex_watch_sync_interval_min is a config.py default read by the watch-sync scheduler job
     # (scheduler.py); it is intentionally NOT a runtime admin knob, so it stays out of SPECS.
     # --- TMDB enrichment (E5 S2; optional) — the API key is the only secret ---