Story 1.4: Create CI Pipeline Contract Test Stage

As a developer, I want the CI pipeline to run contract tests as a dedicated stage, So that breaking interface changes are caught before merge.

FRs: FR3 | Sprint: 22

Acceptance Criteria:

• A contract-test CI stage executes npm run test:contract in backend and document-repository
• The stage runs existing Supertest-based route handler tests that verify request/response schemas (tests written in Epics 2 and 5)
• The stage does NOT write new tests — it orchestrates execution of tests written in Epics 2 and 5
• JUnit XML reports are collected as artifacts:reports:junit
• The stage blocks the MR on failure
• Path-based rules:changes trigger only relevant contract tests on MRs