sunrise: scale audio fades to 8.5% of target duration

Replaces the fixed 0.3s fade with SUNRISE_TARGET_SECS * 0.085, so a 10s
clip now fades 0.85s in and out.  The fade scales automatically if the
target duration is tuned later.

daily_sunrise_video.sh

@@ -288,8 +288,8 @@ if $TEST_MODE; then
 else
     final_video="$output_dir/$CAM_NAME-$current_date-daily-sunrise.mp4"
 fi
-fade_dur=0.3
-fade_out=$(echo "scale=2; $SUNRISE_TARGET_SECS - $fade_dur" | bc)
+fade_dur=$(echo "scale=3; $SUNRISE_TARGET_SECS * 0.085" | bc)
+fade_out=$(echo "scale=3; $SUNRISE_TARGET_SECS - $fade_dur" | bc)
 
 # ── Step 3a: mix audio (video copied, not re-encoded) ────────────────────────
 work_video="$sped_video"