Documentation: Data contracts and ownership map between services

[Cedarich]

Complexity: 150 points

Goal
Document the important data contracts between contracts, data-processing, backend, webapp, and mobile, including who owns each surface.

Acceptance Criteria

• Defines major producer and consumer boundaries.
• Identifies ownership for critical datasets and APIs.
• Highlights where schema or event versioning must stay compatible.
• Useful for planning Wave 7 and later changes safely.