feat: forbid concurrent paperlessngx-backup cronjob runs

[fty4]

Summary

• Add concurrencyPolicy: Forbid to the paperlessngx-backup CronJob so overlapping backup runs can't clobber the shared FTP upload if a run ever exceeds the schedule interval.
• Bump chart version 1.1.0 -> 1.1.1.

Why this PR exists

Closes the verification gate for #4 (end-to-end release pipeline check). Merging this to main is the real trigger for release.yaml: chart-releaser should create a GitHub Release for paperlessngx-backup-1.1.1, update gh-pages's index.yaml, and regenerate the chart table in gh-pages's README, while leaving CNAME and artifacthub-repo.yml untouched.

Test plan

• helm lint charts/paperlessngx-backup passes
• helm template confirms concurrencyPolicy: Forbid renders on the CronJob spec
• CI (Lint and Test Charts) passes on this PR
• After merge: confirm release.yaml run creates GitHub Release paperlessngx-backup-1.1.1 with the packaged chart attached
• After merge: confirm gh-pages's index.yaml gains the new entry
• After merge: confirm gh-pages's README chart table updates to 1.1.1
• After merge: confirm gh-pages's CNAME and artifacthub-repo.yml are unchanged