feat(plex): an off-by-default loudness normalisation toggle for playback

Bug hunt 2026-09, item 14 — "how does the multichannel downmix work, and is it
normalised?" It isn't: `plex/stream.py` has always encoded `-c:a aac -ac 2 -b:a
192k`, i.e. fixed stereo with ffmpeg's default downmix matrix and no loudness
handling at all, which is why a film mastered for a cinema arrives with whispered
dialogue and deafening action.

New admin knob `plex_audio_loudnorm` (Configuration → Plex), DEFAULT OFF: it makes
a film markedly louder and narrows its dynamic range, which is a taste call rather
than a fix. It sits at the end of the Plex group, after the software-transcode
knobs — it is an audio concern, not one of the libx264 ones.

  -af aformat=channel_layouts=stereo,loudnorm=I=-16:TP=-1.5:LRA=11,aresample=48000

Three things about that chain are load-bearing and each has a test:

  * the downmix is FORCED FIRST rather than left to `-ac 2` — measuring 5.1 and
    folding it down afterwards would normalise something other than what is heard;
  * the trailing resample is not decoration: `loudnorm` runs its own resampler at
    192 kHz internally, which the AAC encoder will not take;
  * one `-af`, no stream index, so EVERY audio rendition of a multi-audio file is
    normalised — not just the first.

The knob is also part of the session directory name (`…_multi-ln_+0.00`), because
it changes the audio the segments contain, and a session left over from before the
admin flipped it must not be mistaken for one that honours it. The suffix is
appended with a DASH: `_SESSION_DIR_RE` treats the tag as one underscore-free
field, and that regex is the only thing keeping the orphan sweep off what may be a
shared scratch path. A test pins both halves — the name changes, and the sweep
still recognises it.

Measured end-to-end on the app's own HLS segments (a real 5.1 AC3 file, same item,
same 160 s offset, same ten segments): OFF -35.2 LUFS, ON -17.2 LUFS. On a 60 s
bench: I -27.6 → -16.6 LUFS with LRA 13.3 → 9.8 LU. A/V sync is untouched — under
this pipeline's own `-copyts` the audio start PTS is identical with and without the
filter, so the 3 s lookahead desyncs nothing.

Direct-played files are served untouched and stay so; a 5.1 mix is always `remux`,
so the reported case is fully covered.

backend/app/config.py

@@ -271,6 +271,11 @@ class Settings(BaseSettings):
     # trades quality for size (lower = better + bigger). `max_height` caps the SOFTWARE transcode's output
     # height (0 = keep source) so a CPU re-encode of a tall source stays affordable — the GPU path always
     # encodes at source resolution.
+    # Loudness normalisation of the downmixed stereo audio on the HLS path (EBU R128 `loudnorm`).
+    # OFF by default: it makes a film ~11 dB louder and narrows its dynamic range, which is a taste
+    # call, not a fix. Only affects files that go through remux/transcode — a direct-played file is
+    # served untouched, but those are browser-native stereo anyway, not the 5.1 mixes this is for.
+    plex_audio_loudnorm: bool = False
     plex_transcode_preset: str = "veryfast"
     plex_transcode_crf: int = 20
     plex_transcode_max_height: int = 1080