feat(ops): scheduled Postgres backups via db-backup sidecar

All durable state (library, monitor/wanted lists, discovery, config, and the
encrypted credentials) lived only in the single postgres-data volume with no
backup — one volume loss = total loss. Adds a db-backup compose sidecar that
pg_dumps immediately on start then every BACKUP_INTERVAL_SECONDS (default daily),
keeping the last BACKUP_KEEP (default 7) custom-format dumps in ${BACKUP_DIR}.

Travels with the stack (no host cron), starts with `up -d`. .env.example
documents the knobs; README adds a "Backup & restore" section with pg_restore
steps and an off-box note. Verified live: sidecar wrote a valid 128K dump of the
real lyra DB (pg_restore -l lists all tables + _prisma_migrations).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -41,3 +41,4 @@ Thumbs.db
 .vscode/
 *.swp
 slskd-downloads/
+backups/