Adds a compose 'backup' sidecar: daily pg_dump of the inboxintel DB into ./backups
(git-ignored), atomic write (.tmp -> rename), rotation after BACKUP_KEEP_DAYS
(default 7). Previously there were NO backups — a bad migration or volume loss
meant total data loss. Restore procedure documented in compose + SECURITY.md.
Verified: compose config parses; a live pg_dump against the staging DB produced a
valid dump over the compose network with the same image/credentials the sidecar uses.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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