docs(env): clarify FFMPEG_BIN usage for audio processing in docs and env example

Update .env.example and environment variable documentation to clarify that
FFMPEG_BIN can override the ffmpeg binary path for general audio processing,
not just audiobooks. Note that it is also used by compute worker Whisper audio
decode in addition to audiobook routes.

.env.example

@@ -72,7 +72,7 @@ S3_BUCKET=
 # COMPUTE_WORKER_URL=http://localhost:8081
 # COMPUTE_WORKER_TOKEN=local-compute-token
 
-# (Optional) Override ffmpeg binary path used for audiobook processing
+# (Optional) Override ffmpeg binary path used for audio processing
 # FFMPEG_BIN=
 
 # (Optional) Test/dev overrides

docs-site/docs/reference/environment-variables.md

@@ -348,7 +348,9 @@ There are no dedicated env vars for these runtime settings.
 
 ### FFMPEG_BIN
 
-Override ffmpeg binary path used for audiobook processing.
+Override ffmpeg binary path used for audio processing.
+
+- Used by audiobook processing routes and compute worker Whisper audio decode.
 
 ## Testing and CI