Make JPEG quality configurable

AGENTS.md

@@ -176,6 +176,7 @@ Runtime:
 - `RECENT_URL_LIMIT`: recent URL count, default `12`.
 - `FAVORITES_PATH`: favorites JSON path.
 - `FAVORITES_LIMIT`: favorites count, default `50`.
+- `JPEG_QUALITY`: default JPEG quality, fallback `7`, clamped `2..18`; lower is better for ffmpeg `-q:v`.
 - `MAX_WS_BUFFER_BYTES`: server-side WebSocket JPEG frame backlog cap, default `2097152`.
 - `MAX_AUDIO_QUEUE_BYTES`: single-mode audio output queue cap, default `16777216`.
 - `MAX_RELAY_BRANCH_QUEUE_BYTES`: relay per-branch compressed-input queue cap, default `16777216`.
@@ -184,7 +185,7 @@ Session playback options are accepted by `POST /api/session` even though the UI
 
 - `fps`: default `24`, clamped `1..30`.
 - `width`: default `960`, clamped `160..1920`.
-- `quality`: default `5`, clamped `2..18`; lower is better for ffmpeg `-q:v`.
+- `quality`: defaults to `JPEG_QUALITY`, clamped `2..18`; lower is better for ffmpeg `-q:v`.
 - `audioBitrate`: default `160k`, accepts two or three digits followed by `k`.
 
 ## Docker Notes

README.md

@@ -71,7 +71,7 @@ The UI intentionally hides these settings, but the backend still supports them t
 
 - Frame rate defaults to `24fps`. Lower it if the client cannot keep up.
 - Max width defaults to `960px`. Lower it first if bandwidth or image decode is the bottleneck.
-- JPEG quality uses ffmpeg's `-q:v` scale, where lower is better. `5` is the default, `2` is high quality, and `18` is rough but lighter.
+- JPEG quality uses ffmpeg's `-q:v` scale, where lower is better. Set the default with `JPEG_QUALITY`; `7` is the fallback, `2` is high quality, and `18` is rough but lighter.
 - Audio defaults to MP3 at `160k`.
 
 ## Tradeoffs

docker-compose-example.yml

@@ -15,6 +15,7 @@ services:
       PLAYBACK_CONNECTION_MODE: relay
       FFMPEG_LOG_LEVEL: warning
       FFMPEG_INPUT_SEEKABLE: "0"
+      JPEG_QUALITY: "7"
       MAX_WS_BUFFER_BYTES: "2097152"
       MAX_AUDIO_QUEUE_BYTES: "16777216"
       MAX_RELAY_BRANCH_QUEUE_BYTES: "16777216"

server/index.js

@@ -39,7 +39,7 @@ const JPEG_EOI = Buffer.from([0xff, 0xd9]);
 const defaults = {
   fps: 24,
   width: 960,
-  quality: 5,
+  quality: clampInteger(process.env.JPEG_QUALITY, 7, 2, 18),
   audioBitrate: '160k',
 };