4-seasons: protect night frames from grey filter with brightness floor

A stddev-only filter would wrongly drop near-black night frames.
Now only skips a frame when BOTH stddev < GREY_MIN AND mean brightness
> GREY_DARK_FLOOR (default 30/255).  Night frames are dark so they
always pass; bad-signal grey frames are mid-bright so they are caught.

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

sky-cam.conf

@@ -257,13 +257,21 @@ AUDIO_BITRATE=192k       # montage-mvt.sh           — music track on movement
 
 # ── Frame validation (4-seasons.sh) ──────────────────────────────────────────
 # Corrupt or empty frames (e.g. first frame after a camera reconnect) are always
-# skipped automatically via ffprobe.
+# skipped automatically via ffprobe.  No config needed.
 #
 # SEASONS_GREY_STDDEV_MIN: also skip uniform-grey frames caused by a bad RTSP
-# signal.  These are valid JPEGs but contain a near-solid grey image.
-# Value is the minimum pixel standard deviation on a 0–255 scale.
-# 0 = disabled (default).  Try 5–10 if your camera produces grey frames on reconnect.
+# signal (valid JPEGs, but near-solid grey — no real image content).
+# A frame is only skipped when BOTH conditions hold:
+#   1. pixel standard deviation (0–255) is below SEASONS_GREY_STDDEV_MIN
+#   2. mean brightness (0–255) is above SEASONS_GREY_DARK_FLOOR
+# This keeps genuine night frames (dark + low stddev) while dropping
+# bad-signal grey frames (mid-bright + low stddev).
+#
+# SEASONS_GREY_STDDEV_MIN=0 disables the filter entirely (default).
+# Try 5–10 if your camera produces solid-grey frames on reconnect.
+# Requires ImageMagick (convert) to be installed.
 SEASONS_GREY_STDDEV_MIN=0
+SEASONS_GREY_DARK_FLOOR=30   # frames with mean brightness below this are always kept
 
 # ── Montage attribution overlay ───────────────────────────────────────────────
 # A translucent bar across the top of each movement montage, naming the