[Admin Console] Phase 3: Visual Settings Editor #633

Editing settings.yaml requires SSH access and a text editor. There is no validation before applying changes, no backup before overwriting, and no audit trail of what changed and when.

• GET /admin/settings:- read and return current settings as JSON
• PUT /admin/settings:- validate, back up, and atomically write updated settings
• Schema-aware validation using joi before any write
• Timestamped backup of the previous settings file before each write
• Audit log entry for every applied change
• Trigger the existing hot-reload path so changes apply without a relay restart