Architecture decision records

[erikhfj]

Write up the important choices so the peer reviewers (and the examiners) can follow our reasoning.

See docs/architecture.md. Must cover at least:

• why PostgreSQL + GraphQL Yoga and not MongoDB / Neo4j / a BaaS
• why keyset (cursor) pagination and not OFFSET
• why sorting and filtering live in SQL
• how N+1 is prevented (DataLoaders)
• simulated users instead of login
• plain CSS + Apollo Client cache + URL as search state

Acceptance: each decision has context, options considered, and consequences.

[erikhfj]

Before you open the PR: implement from the requirement, not by copying from archive/scaffold-prototype. The prototype is a reference you can check against when you get stuck. It is not a source to paste from.

The PR description must say what was done and what was tested. That is what we read when we review.