docs: clarify distributed networking setup

[LouisDeconinck]

Summary

• Explain the loopback default and direct Tailnet binding requirement.
• Document the Tailscale Serve EOF symptom and raw TCP fix.
• Add coordinator-initiated reverse SSH forwarding.
• Clarify health-port disabling and the coordinator health bind behavior.

References dagucloud/dagu#2623

Checks

• pnpm build
• pnpm test:worker
• git diff --check

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Summary by cubic

Clarifies the distributed networking docs around loopback defaults, health endpoints, and Tailscale/SSH forwarding.

• Documents that worker.health-port=0 and coordinator.health-port=0 disable only the local HTTP health endpoint, not gRPC traffic; the coordinator health server binds :8091 independently of coordinator.host.
• Explains that the coordinator gRPC bind defaults to 127.0.0.1, so Tailnet membership alone does not expose it; set --coordinator.host=0.0.0.0 or bind to the Tailscale interface for direct access.
• Adds a coordinator-initiated SSH reverse port forwarding setup for workers the coordinator can reach.
• Ties the error reading server preface: EOF symptom to Tailscale Serve using HTTP/HTTPS mode and points to the raw TCP fix.

^{Written for commit f0ee235. Summary will update on new commits.}

[coderabbitai]

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