docs: add server-side gateway actor attribution

[benpsnyder]

Refs #5004

Summary

• Adds a server-side gateway auth guide covering actor attribution for per-user, service/robot, and hybrid topologies.
• Documents when reducers should trust native ctx.sender versus a gateway-asserted effective actor.
• Adds a TypeScript reducer example that validates a gateway token before accepting delegated actor input.
• Links the new guide from the Authentication overview.

Validation

• git diff --check
• pnpm --dir docs typecheck
• pnpm --dir docs build

Note: Docusaurus build completes successfully but still emits the existing llms-txt warning for /docs/ask-ai/ask-ai.

[cloutiertyler]

Hi @benpsnyder, thanks for your contribution to the docs. We don't accept draft PRs in our repo to keep our PRs relatively neat and manageable, so I'm going to close this for now, at least until you have a complete PR for us to review.