fix(ci): provide auth secret during release

[dancormier]

Summary

• Adds a release-job BETTER_AUTH_SECRET fallback so Better Auth does not abort docs builds during release.
• Matches the placeholder secret behavior already used by the reusable build/test workflow.

Testing

• git diff --check
• Not run: npm run build locally because npm is not available on this shell's PATH.

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Latest commit: c25400a

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