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>

.env.example

@@ -0,0 +1,6 @@
+# Webhook for update/backup notifications (n8n → Telegram)
+WEBHOOK_URL=https://n8n.example.com/webhook/your-webhook-id
+
+# Restic backup repository and password
+RESTIC_REPOSITORY=/mnt/hidrive/users/valknar/Backup/stacks
+RESTIC_PASSWORD=changeme