feat: compress MP4 videos at Docker build time via FFmpeg

Hugo has no video processing, so FFmpeg runs in the builder stage
before hugo builds, compressing every content/**/*.mp4 in-place.

scripts/compress-videos.sh:
  - H.264 CRF 28 + preset slow (good web compression)
  - faststart for progressive streaming
  - AAC 64k audio
  - Only replaces source if output is actually smaller, so
    already-lean videos are skipped

Dockerfile:
  - apk adds ffmpeg alongside hugo in the builder stage
  - RUN compress-videos.sh runs after COPY . . before pnpm build
    (compressed files land in public/ via Hugo's copy of page bundles)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/compress-videos.sh

@@ -0,0 +1,47 @@
+#!/bin/sh
+# Compress MP4 page-bundle videos with FFmpeg before Hugo builds.
+#
+# Usage: sh scripts/compress-videos.sh [content-dir]
+#
+# Encoding choices:
+#   H.264 CRF 28  — good web quality, ~60-80% smaller than camera/AI originals
+#   preset slow   — best compression for a given CRF (fine in CI/Docker)
+#   faststart     — moov atom moved to front for progressive streaming
+#   aac 64k       — low-bitrate audio (videos are muted in-player by default)
+#
+# The script replaces each source file in-place only when the compressed
+# output is actually smaller, so already-lean videos are left untouched.
+
+set -e
+
+CONTENT_DIR="${1:-content}"
+
+find "$CONTENT_DIR" -name "*.mp4" | while IFS= read -r src; do
+  tmp="${src%.mp4}.__tmp.mp4"
+
+  echo "▶ $src"
+
+  ffmpeg -i "$src" \
+    -c:v libx264 \
+    -crf 28 \
+    -preset slow \
+    -movflags +faststart \
+    -c:a aac \
+    -b:a 64k \
+    -loglevel error \
+    -y "$tmp"
+
+  src_size=$(stat -c%s "$src" 2>/dev/null || stat -f%z "$src")
+  tmp_size=$(stat -c%s "$tmp" 2>/dev/null || stat -f%z "$tmp")
+
+  if [ "$tmp_size" -lt "$src_size" ]; then
+    mv "$tmp" "$src"
+    saved=$(( (src_size - tmp_size) / 1024 ))
+    echo "  ✓ ${src_size} → ${tmp_size} bytes  (−${saved} KB)"
+  else
+    rm -f "$tmp"
+    echo "  — already optimal, skipped"
+  fi
+done
+
+echo "Done."