refactor: absorb _backup and _update into stacks.sh

- Inline update logic (pull → compare digests → up -d → prune → notify)
- Inline backup logic with dynamic Postgres detection: any running
  <stack>_db container is dumped using the <stack>/<stack> convention
- Systemd unit files are now generated on `install` from embedded
  heredocs pointing at stacks.sh itself — no external scripts needed
- Root .env (WEBHOOK_URL, RESTIC_REPOSITORY, RESTIC_PASSWORD) replaces
  the per-service .env files in _backup/ and _update/
- Remove _backup/ and _update/ directories entirely
- Update README accordingly

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

_backup/stacks-backup.service

@@ -1,10 +0,0 @@
-[Unit]
-Description=Backup stacks data to HiDrive via restic
-After=network-online.target docker.service
-Wants=network-online.target
-
-[Service]
-Type=oneshot
-User=root
-ExecStart=/root/stacks/_backup/backup.sh
-Environment=HOME=/root