chore(deps): add Dependabot configuration

[tigdav]

Summary

• Added .github/dependabot.yml to enable automated dependency update PRs.
• Configured two ecosystems on a weekly schedule: pip (Python deps under /backend) and github-actions (workflow actions).

Notes

• Configuration-only change. No application code, settings, or CI workflow changes.
• Commit-message prefixes are aligned with this repo's convention (chore(deps) for Python, ci(deps) for Actions).
• Dependabot will surface security and version updates as individual PRs, complementing the recent manual Django 5.2 / Postgres 16 upgrades.