Connect events and next= on the OAuth door

[czpython]

Two additions to the Connections flow.

Connect events

Extensions that keep per-connection state (a sync cursor, an enable flag)
had no way to hear about a new sign-in — the callback stored the row and
rendered a terminal page, leaving apps to sweep list_for_provider on a
cron. The platform now publishes on the existing signal machinery:

• oauth.connected — after the callback stores a connection, both
  branches; reconsent is true when it replaced an existing connection's
  tokens. Fields: provider, connection_id, account_id, reconsent.
• oauth.disconnected — when a connection dies: revoked from the
  Connections page, or purged because the service's client credentials
  were replaced. Same fields, no reconsent.

@subscribe("oauth.connected", provider="acme")
async def adopt_sign_in(
    provider: str, connection_id: str, account_id: str, reconsent: bool
) -> None:
    ...

next= on the connect door

GET /api/oauth/{name}/connect?next=/app/night_watch/accounts rides
the pending-state stash; a successful callback redirects there instead of
rendering the "connected" page, so an app's sign-in button lands the user
back on its own page. Only a bare same-origin path is accepted — it must
start with / and anything host-shaped (//…, /\…, a scheme) is
rejected with 422, so the door cannot become an open redirect.

Both are documented next to the connections section in
writing-an-extension.md.