[WIP] Fix E2E workflow Keycloak host port configuration

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original prompt

Fix failing E2E workflow where Keycloak is started on the wrong host port.

Context:

• GitHub Actions job: https://github.com/ls1intum/thesis-management/actions/runs/22567341438/job/65366325416
• The workflow .github/workflows/e2e_tests.yml starts Keycloak with -p 8081:8080 but subsequent steps (wait-for-ready, realm import, and env vars for server/client) use http://localhost:8181.
• Team intent: E2E tests and local development should use port 8181; production should remain unchanged.

Required changes:

1. In .github/workflows/e2e_tests.yml, update the Keycloak docker run port mapping to use -p 8181:8080 so it matches the health check and KEYCLOAK_HOST URLs.
2. Keep the PR small and focused; do not change production workflows.
3. Ensure workflow still uses the same Keycloak image and command (quay.io/keycloak/keycloak:26.5 start-dev).

Acceptance criteria:

• E2E workflow no longer times out waiting for Keycloak.
• All Keycloak URLs in the E2E workflow are consistent with the mapped port.

This pull request was created from Copilot chat.

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