Document wiki-scoped Keycloak configuration decision

[fpichler]

Context

Records the ALU-190 Keycloak configuration trust boundary for the reusable Celements adapter. See ALU-190 and the Progon review discussion.

Decision

• Wiki-level Keycloak configuration is shared by all Spaces; Space preferences cannot override it.
• Wiki administrators may configure the destination only where the deployment explicitly trusts them with that authority.
• The ADR documents source precedence, the files-only alternative, and the credential-disclosure consequence of this trust choice.

Scope and validation

• Adds ADR 0002 and its index entry only; ADR 0001 and source code are unchanged.
• Markdown links and index entry verified; git diff --check passed.
• Independent standards and specification review approved the complete change.

This PR is stacked on #313, which introduces ADR 0001 and the ADR index. After #313 merges, retarget this PR to dev.