Add per-step resilience and notifications across all video pipeline scripts

Each pipeline now saves intermediate files permanently before the step most
likely to fail, so a partial failure leaves a recoverable artifact:
- daily_sunrise_video.sh: speed-adjusted video (*-sped.mp4) survives if the
  drawtext overlay step fails; deleted automatically on success
- montage-mvt.sh: speed-adjusted video (*-Sped.mp4) survives if the
  music-mux + attribution overlay step fails; deleted on success
- 4-seasons.sh / montage-mvt.sh / year-end-join.sh: explicit ffmpeg error
  handling with targeted notify.sh calls naming the failed step and the
  surviving artifact path
- montage-mvt.sh / 4-seasons.sh / year-end-join.sh: clip-duration-sum vs
  music-duration drift check warns when cumulative float error exceeds 2 s

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

year-end-join.sh

@@ -32,6 +32,7 @@ concat_list=$(mktemp --suffix=.txt)
 trap 'rm -f "$concat_list"' EXIT
 
 missing=0
+total_expected_dur=0
 for season in Winter Spring Summer Autumn; do
     for mvt in 1 2 3; do
         mvt_dir="$year_dir/$season/Mvt$mvt/montage"
@@ -41,11 +42,14 @@ for season in Winter Spring Summer Autumn; do
             echo "WARNING: missing montage for $season Mvt$mvt ($mvt_dir)"
             missing=$((missing + 1))
         else
-            echo "  $season Mvt$mvt → $(basename "$f")"
+            d=$(ffprobe -v error -show_entries format=duration -of csv=p=0 "$f")
+            total_expected_dur=$(echo "scale=3; $total_expected_dur + $d" | bc)
+            echo "  $season Mvt$mvt → $(basename "$f")  (${d}s)"
             printf "file '%s'\n" "$f" >> "$concat_list"
         fi
     done
 done
+echo "Expected total duration: ${total_expected_dur}s ($(echo "scale=1; $total_expected_dur/60" | bc) min)"
 
 found=$(wc -l < "$concat_list")
 echo "$found of 12 movements found  ($missing missing)"
@@ -63,14 +67,26 @@ fi
 output_file="$year_dir/${CAM_NAME}-${ASTRO_YEAR}.mp4"
 echo "Output: $output_file"
 
-ffmpeg -loglevel warning \
+if ! ffmpeg -loglevel warning \
     -f concat -safe 0 -i "$concat_list" \
-    -c copy -y "$output_file"
+    -c copy -y "$output_file"; then
+    "$SCRIPT_DIR/notify.sh" "FAILED: year-end join $CAM_NAME $ASTRO_YEAR" \
+        "ffmpeg concat failed — the 12 movement montages are still intact" || true
+    exit 1
+fi
 
 total_dur=$(ffprobe -v error -show_entries format=duration -of csv=p=0 "$output_file")
 total_min=$(echo "scale=1; $total_dur / 60" | bc)
 echo "Done: $output_file  (${total_min} min)"
 
+# Sanity-check actual vs expected duration
+dur_drift=$(echo "scale=3; $total_dur - $total_expected_dur" | bc | sed 's/^-//')
+if awk "BEGIN{exit !($dur_drift > 2.0)}"; then
+    echo "WARNING: output ${total_dur}s differs from expected ${total_expected_dur}s by ${dur_drift}s"
+    "$SCRIPT_DIR/notify.sh" "WARNING: year-end duration drift $CAM_NAME $ASTRO_YEAR" \
+        "Output ${total_dur}s vs expected ${total_expected_dur}s (drift ${dur_drift}s)" || true
+fi
+
 # ── Notify ────────────────────────────────────────────────────────────────────
 "$SCRIPT_DIR/notify.sh" \
     "Four Seasons $ASTRO_YEAR complete" \