4-seasons: validate JPEGs with ffprobe, add optional grey-frame filter

The previous size check caught empty files but not corrupt JPEGs with
a partial header (common first frame after a camera reconnect).
Replace with ffprobe codec detection — if ffprobe can't identify a
video stream in the file it's skipped with a clear "corrupt" message.

Also adds SEASONS_GREY_STDDEV_MIN (default 0 = off): when enabled,
frames with near-zero pixel standard deviation (uniform grey = bad
RTSP signal) are silently dropped before encoding.

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

sky-cam.conf

@@ -255,6 +255,16 @@ CRF_MONTAGE_FINAL=22     # montage-mvt.sh           — final archive montage (H
 ENCODE_PRESET=slow       # ffmpeg preset for all encodes (slow/medium/fast)
 AUDIO_BITRATE=192k       # montage-mvt.sh           — music track on movement montages
 
+# ── Frame validation (4-seasons.sh) ──────────────────────────────────────────
+# Corrupt or empty frames (e.g. first frame after a camera reconnect) are always
+# skipped automatically via ffprobe.
+#
+# 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.
+SEASONS_GREY_STDDEV_MIN=0
+
 # ── Montage attribution overlay ───────────────────────────────────────────────
 # A translucent bar across the top of each movement montage, naming the
 # music source ("The Four Seasons — Antonio Vivaldi ...").