Add fixture-based test data for the Prisma and integration test suites

[robertocarlous]

Why
The current tests would benefit from reusable fixtures that represent common users, sessions, event rows, and DLQ entries. Shared fixtures would reduce duplication and make edge cases easier to add.

Acceptance criteria

• Create reusable fixtures for the major database entities.
• Use them across unit and integration tests where practical.
• Keep fixture creation deterministic and easy to reset.