ADR-0001/5: AGENTS.md + test-audit.md update (single-field uniqueness prose)

[TheBoomerDev]

Summary

Update backend/AGENTS.md to reflect the new email-uniqueness rule (single-field)
instead of the obsolete compound description. Update backend/docs/test-audit.md
with the new test cases from issue D.

Spec

The decision lives at https://github.com/cargoffer/education_backend/blob/feat/adr-0001-global-email-uniqueness/docs/adr/0001-global-email-uniqueness.md. Maps to § Implementation Plan → 5. Documentation.

Acceptance criteria

• backend/AGENTS.md:
  • Replace any prose referring to "compound (email, regionId) UNIQUE" with the new single-field rule. Cite the ADR.
• backend/docs/test-audit.md:
  • Add the new auth-contract cross-region duplicate test (issue D) to the inventory table.
  • Add the precision migration test (issue D) to the inventory.
• backend/src/scripts/seed.ts and scripts/audit-email-duplicates.ts (issue A) cross-references are added where relevant.

Out of scope

• Frontend translation keys, OpenAPI updates.

Dependencies

• BLOCKED BY issues A–D.
• BLOCKS issue F (PR).

References

• ADR: https://github.com/cargoffer/education_backend/blob/feat/adr-0001-global-email-uniqueness/docs/adr/0001-global-email-uniqueness.md — §Implementation Plan → 5