[NFR-03] Prefer open-source technologies where feasible

[manuelgrabmayer]

Requirement:
All infrastructure components shall preferably use open-source technologies.

Goal:
Reduce vendor lock-in and keep the prototype cost-efficient and maintainable.

Acceptance criteria:

1. Main technology choices are documented.
2. Open-source alternatives are considered for core components.
3. Any paid or proprietary tooling is justified.
4. The final tech stack is aligned with project constraints.