backup: nightly pg_dump service on the postgres:18 image

Production Postgres had NO automated backup (pg_dump appeared only in the
one-time migration script). New db-backup service in both compose files fires
20:10 ET Mon-Fri: pg_dump | gzip with pipefail + gunzip -t + tmp-then-rename
(a mid-stream death can't masquerade as a backup), prunes past
BACKUP_KEEP_DAYS, writes last_backup_ok on success. postgres:18 image =
version-matched client (app image has none). TrueNAS host is pull-only, so
its copy inlines the script. Restore: gunzip -c dump | psql $DATABASE_URL.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.env.example

@@ -51,6 +51,11 @@ TELEGRAM_CHAT_ID='2082795871'
 DEADMAN_STALE_MIN=5   # alert when no events for this many minutes during market hours
 DEADMAN_DAILY_PING=1  # nightly OK-ping after 20:30 ET — proves the watchdog itself is alive
 
+# Nightly pg_dump (db-backup compose service, 20:10 ET Mon-Fri) — the only
+# automated backup of the production Postgres. See deploy/docker/backup_db.sh.
+BACKUP_DIR=/data/backups  # inside the db-backup container (app-data volume)
+BACKUP_KEEP_DAYS=14       # prune dumps older than this many days
+
 #Market Data these are free plans so work within days/months capacity.
 Alpha_Vantage_Api_Key=<REDACTED>
 Twelve_Data_Api_Key=<REDACTED>