feat(mailbox): default mountMailbox's SSE bus to in-process (CL-8775)

[TheGreatAxios]

Split out of #28 so that PR stays docs-only.

MountMailboxOpts.bus becomes optional and defaults to createInMemoryMailboxEventBus(). The bus only carries SSE live updates; mail itself is Postgres. A single-process host can omit it; several hub processes must still pass a shared bus, or live updates won't cross processes.

Open question before merging: this is a silent default. A multi-process host that forgets bus loses cross-process live updates with no error.

Once #28 lands, the README's bus row needs to say optional.

[TheGreatAxios]

Closing: bus stays required. The default would fail silently in the one case that matters — a host running several hub processes that forgets to pass a shared bus gets working mail but live updates that only reach browsers on the same process, with no error. A required argument can't be forgotten, and the saving is one line per host. If the convenience is wanted later, the safer shape is an explicit opt-in (e.g. bus: inProcessBus()) rather than a silent default.