Unit/integration/contract tests

[fabianabarca]

Description

Test endpoints and OpenAPI conformance.

Acceptance Criteria

• Unit tests for serializers/validators
• Integration with DB/Redis
• Contract tests vs OpenAPI

[fabianabarca]

Tracked in #26

[RichardCMX]

Starts: 2025-11-10
Ends: 2025-11-23
Depends on: #32, #31, #30, #29, #28 and #27