docs: clarify OIDC middleware guidance

[dlukt]

Summary

• expand the authentication guide with explicit guidance on using the generated OIDC middleware
• document a concrete viewer example that maps the sub claim and provide an up-to-date custom mapper sample
• annotate the generated claims structs so Subject is clearly tied to the sub claim

Testing

• go test ./... (aborted after module downloads due to extended runtime)

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https://chatgpt.com/codex/tasks/task_b_68f2207332f4832e891662399ad5e872