deploy: write the books compose override in the deploy account home

Run #79 failed at "sudo tee /opt/personal-website/compose.books.yml": the deploy
account may only sudo-tee .env. Write the override to ~deploy instead; the
compose CLI runs as root and reads it from there.

README.md

@@ -255,7 +255,7 @@ Files are read from `BOOKS_ROOT` (default `/books` when it exists, otherwise `~/
 - `BOOKS_ROOT/library/<books.files.path>` for book files (written by `scripts/books-ingest.py`)
 - `BOOKS_ROOT/covers/<books.works.cover_path>` for downloaded covers
 
-In production, `scripts/deploy-remote.sh` writes `/opt/personal-website/compose.books.yml`, which
+In production, `scripts/deploy-remote.sh` writes `~deploy/compose.books.yml` (the deploy account may only sudo-tee `/opt/personal-website/.env`), which
 bind-mounts the host's `BOOKS_HOST_DIR` (default `/home/stan/books`) read-only at `/books` and sets
 `BOOKS_ROOT=/books`. Without a `library` directory there the deploy still succeeds, but files can't
 be opened.