docs: document HLS_COPY_MAX_BITRATE

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

.env.example

@@ -29,6 +29,9 @@ EXCLUDED_DIRS=
 # ── Video / HLS ─────────────────────────────────────────────────────────
 HLS_CONCURRENCY=2
 HLS_TIMEOUT_SECONDS=900
+# Sources above this bitrate (bits/s) or 1080p are transcoded instead of
+# stream-copied, so HLS segments stay small enough to start and seek quickly.
+HLS_COPY_MAX_BITRATE=20000000
 PLAYLIST_CLEANUP_INTERVAL_SECONDS=86400
 
 # ── Telemetry ────────────────────────────────────────────────────────────