feat(client): implement OIDC flow

[BjarneHa]

What does this PR do?

Implements basic OIDC flow for the client. Closes #6.

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Docs
• CI / infra

Definition of Done

• CI is green
• If the API changed: api/openapi.yaml is updated and lint passes
• If a service was added or restructured: docker-compose still comes up cleanly (docker compose up)
• Tests added or updated for the changed behaviour
• Docs updated where it matters (README, ADRs, comments)

Notes for the reviewer

Should the development server live in the docker-compose.yaml which we use for production? Instead of using profiles, we could also provide a separate docker-compose.dev.yaml.

[ladu-tu]

Regarding the docker-compose.yaml: I would not add a second compose file again, we just recently reduced it to a single one in PR #148. I would suggest just sticking to profiles, or is there any significant downside to that?