4-seasons: add optional date argument

Allows processing any past date instead of always yesterday:
  ./4-seasons.sh east 2026-04-19

Useful for backfilling days with existing images.
Systemd (no date arg) continues to default to yesterday.

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

4-seasons.sh

@@ -14,10 +14,13 @@ source "$SCRIPT_DIR/sky-cam.conf"
 export TIMEZONE   # make it visible to season_info.py subprocess
 
 # Camera name: required first argument (systemd passes it via ExecStart).
+# Date: optional second argument (YYYY-MM-DD); defaults to yesterday.
 CAM_NAME="${1:-}"
+DATE_ARG="${2:-}"
 if [ -z "$CAM_NAME" ]; then
-    echo "Usage: $0 <camera-name>"
-    echo "  e.g. $0 east   or   $0 north"
+    echo "Usage: $0 <camera-name> [YYYY-MM-DD]"
+    echo "  e.g. $0 east"
+    echo "       $0 east 2026-04-20"
     exit 1
 fi
 
@@ -27,7 +30,11 @@ music_base_dir="$MUSIC_DIR"
 [ ! -d "$music_base_dir" ] && music_base_dir="$SCRIPT_DIR/music"
 
 # ── Date setup ────────────────────────────────────────────────────────────────
-yesterday=$(date --date="yesterday" +%Y-%m-%d)
+if [ -n "$DATE_ARG" ]; then
+    yesterday=$(date --date="$DATE_ARG" +%Y-%m-%d)
+else
+    yesterday=$(date --date="yesterday" +%Y-%m-%d)
+fi
 echo "Processing: $yesterday"
 
 # ── Season / movement info from astronomical calculation ──────────────────────