Implement API Deprecation Strategy

[ONEONUORA]

Component: Backend | Effort: 2-3 days | Priority: MEDIUM

Description

No deprecation strategy. Need to manage API changes gracefully.

Tasks

• Define deprecation policy
• Add deprecation headers
• Create deprecation timeline
• Document deprecated endpoints
• Add deprecation warnings
• Test deprecation flow

Acceptance Criteria

• Deprecation policy documented
• Deprecated endpoints marked
• Clients notified of deprecations