chore: remove closed issue documentation files

Removed 6 closed issue documentation files from docs/issues/:
- #309: epic-add-backup-support (completed)
- #310: research-database-backup-strategies (completed)
- #315: implement-backup-support (completed)
  - phase-1.1b-manual-testing
  - phase-1.1b-results
  - phase-4-documentation-and-testing-plan

All these issues have been closed on GitHub with status 'completed' and are
no longer active work items.

Remaining open issues: #250, #252, #274, #304

Closed issues remain available in git history if needed for future reference.

Author: josecelano