Task: Fix Spring Service Document API

[ladu-tu]

What does this PR do?

This PR is related to issues reported by @BjarneHa regarding file uploads failing. This PR splits up bidirectional references in the JPA schema, which previously lead to an error due to unbounded recursion. Additionally this PR includes some improvements for text extraction (remove null bytes which can't be stored by PostgreSQL), user lookup (overwrite oicdSubject if it changed, e.g. because of a different keycloak session) and the GenAI interaction (explicit content type) that were made during debug.

Furthermore, documentation for the spring service was added under services/spring/README.md and additional test cases for the KnowledgeBase related classes have been added.

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

[BjarneHa]

Uploading documents now works but GET /api/v1/knowledgebase/documents/ still yields a HTTP 500 with {"code":"INTERNAL_ERROR","message":"Internal server error"} after a document has been uploaded.

[BjarneHa]

Nice work!