docs: reference pages for data models, SSE events, and env vars

[bertheto]

Summary

• Add three MkDocs reference pages: data models, SSE events, and environment variables (DOC-08/09/10).
• Register pages in mkdocs.yml under Reference after REST API.
• Sources: TypeScript models.ts, errors.ts, memoryStore/server.ts event bus, and registry.ts config.

Test plan

• python -m mkdocs build succeeds from repo root
• New pages appear in Reference nav
• Readthedocs preview (post-merge)