Add operational lifecycle signals

[knifecake]

Summary

• add public ProcessLifecycle and QueueLifecycle sender types
• emit process start, stop, and restart signals with stable operational payloads
• emit queue pause and resume signals, including whether state actually changed
• keep internal runtime and model objects out of the public signal contract
• prevent forked children from falsely emitting supervisor shutdown events
• document the signals and add regression coverage, including a real-fork probe

Test plan

• make lint
• full PostgreSQL suite: 171 tests passed
• focused observability signal suite: 5 tests passed
• Sphinx documentation build

Closes #23

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-06T21:04:27.166516Z	bba33ae	PR opened

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: bba33aeac7

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