docs: add data-model and auth-flow diagrams and handoff notes

[kevinrutledge]

Developer

Kevin Rutledge

What changed?

The architecture doc had three flowcharts but no data-model or auth diagram, and the repo-access handoff was undocumented. This adds two Mermaid diagrams and folds a handoff section into the future-work doc.

• docs/figures/data-model.mmd + .png - an ER diagram of the 12 Prisma models and their relations
• docs/figures/auth-flow.mmd + .png - a sequence diagram of the portal -> callback -> 303 -> cookie -> DAL login flow
• docs/architecture.md - embed both diagrams, with a note on why the callback uses 303 over 307
• docs/future-work.md - add "Repository access and handoff" (how the next team gets repo admin via an org owner / leads team) and "Remaining co-op handoff items" (referral-tab link, Blob store, test row)

How to test

1. Open docs/architecture.md and confirm the ER diagram renders under Data Model and the sequence diagram under Token Authentication.
2. Open docs/future-work.md and confirm the two handoff sections at the end.
3. To regenerate a diagram: npx -y @mermaid-js/mermaid-cli -i docs/figures/data-model.mmd -o docs/figures/data-model.png -b white.

Checklist

• Code works and is readable
• Tested locally
• Commits follow conventional commits
• Assigned reviewers