Fix Workshop SSE shutdown hang

[dcellison]

Summary

• signal active Workshop SSE handlers when their aiohttp application begins shutdown
• let persistent event streams exit promptly and release their capacity slots
• bound each HTTP runner's defensive graceful-request timeout below the service-generation watchdog
• cover shutdown with a real server holding an otherwise 30-second-idle SSE connection open

Why

An open Workshop browser keeps a persistent SSE request active. During service shutdown, aiohttp waited its default 60-second handler timeout, while Kai's verified generation guard escalated to SIGKILL after 65 seconds. Installs and kickstarts therefore appeared hung and the old process did not complete its cleanup path.

Validation

• 6084 passed in 73.62s
• make check
• make typecheck
• focused HTTP/Workshop suite: 315 passed
• real-server SSE shutdown regression completes in under one second