test: introduce backend regression tests and CI workflow (Phase 1)

[aviralsaxena16]

Hello @1brahimmohamed @jan-petr,

This PR ports the initial regression testing setup and backend CI workflow into the official OSIPI repository.

The original version was implemented and discussed previously, and I am reintroducing it here to establish a structured testing baseline in the canonical repository.

What this PR introduces (Phase 1)

• Backend regression test scaffolding at the FastAPI layer (apps/backend/) using TestClient
• API schema validation test
• Snapshot-based regression tests using pytest-regressions
• Multi-version GitHub Actions CI workflow (Python 3.9–3.11)
• Validation of deterministic JSON outputs
• Basic negative test for invalid DICOM upload

This phase establishes a reproducible safety net for the API surface and end-to-end report generation flow before introducing curated fixtures.

Next Step (Phase 2)

In a follow-up PR, I plan to:

• Integrate minimal curated GE and Siemens fixtures
• Snapshot deterministic outputs from real baseline sequences
• Add PDF endpoint validation
• Ensure stable ordering/normalization for robust regression comparisons

The goal is to evolve this from regression scaffolding into a full deterministic validation framework aligned with the GSoC objective of comparing generated outputs against expected outputs.

Happy to iterate based on feedback.

Best,
Aviral