Introduce DATA_DIR env var for consistent data path configuration

Replace separate DATABASE_URL and IMAGE_CACHE_DIR env vars with a
single DATA_DIR root (default: ./data, Docker: /data). DATABASE_URL
and CACHE_DIR are derived from it but can still be overridden
individually.

Also:
- Pin pnpm to @10 for reproducible Docker builds
- Add --link to COPY instructions for better BuildKit cache reuse
- Add syntax directive for BuildKit features
- Simplify Dockerfile mkdir to just /data (ensureImageDirs handles subdirs)
- Remove redundant DATABASE_URL from docker-compose.yml

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

docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   sofa:
-    image: ghcr.io/jakejarvis/sofa:latest
+    image: ghcr.io/jakejarvis/sofa:edge
     container_name: sofa
     restart: unless-stopped
     ports:
@@ -8,7 +8,6 @@ services:
     volumes:
       - sofa-data:/data
     environment:
-      - DATABASE_URL=file:/data/sqlite.db
       - TMDB_API_READ_ACCESS_TOKEN=${TMDB_API_READ_ACCESS_TOKEN}
       - BETTER_AUTH_SECRET=${BETTER_AUTH_SECRET}
       - BETTER_AUTH_URL=${BETTER_AUTH_URL:-http://localhost:3000}