A suspended account's open browser streams and app connections end, and a push under way is cut #785

Suspending an account (#660) stops every credential and ends its open SSH sessions. Long-lived connections opened before the suspension can still carry on: the forge's live streams (SSE) to its browser pages, an app's websocket through the forge's proxy, and possibly a git push already streaming over SSH.

Acceptance criteria

• [ ] On account.suspended or account.removed, the forge closes that account's open SSE streams and the websockets it proxies for that account to its apps.
• [ ] A git push already under way over SSH or HTTP when the account is suspended is cut, and its ref update is refused.
• [ ] Tests: an SSE stream and an app websocket opened by a fresh account close when it is suspended; a push started before the suspension does not land.

Duplicate of #757.