feat(pitr): periodic pgBackRest catalog verification in backup watcher

[paulocsanz]

Summary

• Adds a periodic (once/hour) check that verifies the pgBackRest catalog agrees with the local state file
• If last_full_at is set but pgbackrest info shows 0 full backups (path mismatch, stanza recreated, etc.), clears last_full_at so NEEDS_INITIAL_BACKUP fires on the next poll
• Probe returns early on any pgbackrest error so transient S3 failures don't incorrectly reset state
• No new dependencies — pure bash/grep
• Complements the backboard-side fix that gates triggerGapRecoveryBackupActivity on the 24h grace period

Test plan

• bash -n pgbackrest-backup-watcher.sh passes
• With a state file containing a stale last_full_at and an empty pgBackRest catalog, watcher resets state and takes a new full within one poll cycle

[paulocsanz]

Superseded by #80 which also includes the matching Rust implementation in postgres-ha (#40).