feat: add unified auth GraphQL integration

[Zetazzz]

Summary

• Add GraphQL operations for unified login start and local authentication.
• Generate opaque transaction identifiers at the server boundary and send only digests to PostgreSQL.
• Bind browser state to the active transaction and preserve existing authentication cookie behavior.

Stack

Base: #1705
Next: #1707

Validation

• targeted GraphQL server tests: 54 passed
• affected package builds passed