Post-migration setup

[m2papierz]

Summary

This PR standardizes and hardens the repository’s GitHub setup and contributor experience.

It aligns CI, dependency management, security reporting, and contribution guidelines with current best practices, ensuring consistency between documentation, workflows, and enforced branch rules.

Scope of changes

• Normalize GitHub configuration files and locations (CODEOWNERS, issue templates, Dependabot).
• Improve CI alignment with documented local workflows (uv, pre-commit, pytest).
• Refine contribution guidelines for clarity and reproducibility.
• Make the PR template more explicit as a quality gate.
• Standardize security reporting by introducing a dedicated security contact.

Impact

• No user-facing functional changes.
• Improves maintainability, security posture, and contributor onboarding.
• Establishes a clean, scalable baseline for an OSS project.