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>

Dockerfile

@@ -1,8 +1,8 @@
 # Stage 1: Build
 FROM node:22-alpine AS builder
 
-# Install Hugo
-RUN apk add --no-cache hugo
+# Install Hugo + FFmpeg (video compression)
+RUN apk add --no-cache hugo ffmpeg
 
 # Install pnpm
 RUN corepack enable && corepack prepare pnpm@latest --activate
@@ -16,6 +16,9 @@ RUN pnpm install --frozen-lockfile
 # Copy source files
 COPY . .
 
+# Compress MP4 videos in-place before Hugo builds
+RUN sh scripts/compress-videos.sh content
+
 # Build CSS and Hugo site
 RUN pnpm build