Phase 6: Publish OpenAPI contract for implemented endpoints

[wauputr4]

Why

Docs already require API contract visibility, but repo still lacks a published OpenAPI artifact and contract-level validation workflow. This makes consumers and integrators less confident for API-first usage.

Scope

• Add OpenAPI generation for implemented public + admin endpoints.
• Keep endpoint list aligned with docs/04-api-cli.md and current router.
• Include request/response schema for posts/impact/orgs/members/relationships/auth/admin list routes.
• Add GET /api/v1/openapi.json endpoint in dev mode (or static docs path).
• Add simple smoke test that verifies one representative contract shape and status code.
• Document where to fetch/publish openapi in docs.

Definition of done

• A consumer can inspect endpoint contracts without reading source.
• CI/CD can fail on broken schema drift in a future step.
• This issue can be referenced from Phase 6 and release notes.