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>

.env.example

@@ -21,3 +21,10 @@ MUSIC_DIR=./music
 # Set this only once Lyra runs on the same host as slskd; until then Soulseek can search but
 # not deliver. Defaults to ./slskd-downloads (an empty local dir) so the stack still starts.
 # SLSKD_DOWNLOADS_DIR=/var/lib/slskd/downloads
+
+# Postgres backups (db-backup sidecar). Dumps go to BACKUP_DIR on the host; the sidecar
+# keeps the last BACKUP_KEEP dumps and runs every BACKUP_INTERVAL_SECONDS (default daily).
+# Point BACKUP_DIR at a NAS/synced path for off-box safety. See README "Backup & restore".
+# BACKUP_DIR=./backups
+# BACKUP_INTERVAL_SECONDS=86400
+# BACKUP_KEEP=7