Align user-association payload adapters with API contracts

Base policy, household, simulation, and report creation payloads now match the API's current creation schemas. The remaining work concerns user-association payloads and endpoints.

• Replace any in household and geographic association adapters with explicit request and response types.
• Resolve inconsistent camel-case and snake-case conversion across association adapters.
• Confirm that each user-*-association endpoint used by the app exists in the intended API version and matches the corresponding database schema.
• Confirm that database-owned fields, including created_at and updated_at, are not generated or sent by the client unless an endpoint explicitly requires them. This absorbs #199.
• Add cross-repository contract tests or generated types so future API changes fail deterministically.