API Versioning Strategy

Design an API versioning strategy for:

**API type:** [TYPE — e.g., "REST API with 50+ endpoints"]
**Consumers:** [CONSUMERS — e.g., "mobile app, web app, third-party integrations"]
**Change frequency:** [FREQUENCY — e.g., "breaking changes every 3-6 months"]

Compare and recommend between:
1. **URL versioning** (/v1/users, /v2/users)
2. **Header versioning** (Accept: application/vnd.api+json;version=2)
3. **Query parameter** (?version=2)

Then generate:
1. **Version routing** — middleware that routes to correct handler version
2. **Deprecation policy** — sunset headers, deprecation timeline, migration guides
3. **Backward compatibility** — additive changes that don't need a new version
4. **Version negotiation** — how clients specify preferred version
5. **Documentation** — OpenAPI spec with multiple version support
6. **Migration tooling** — automated client-side migration helpers

Include the routing middleware and deprecation header implementation.