api: extend contexts and DTOs for order books and event creation

The desktop UI needs to create events, trade on both sides of an order
book, and show who is participating in an event, none of which the api
module could express so far.

Add AccountContext for looking up ledger accounts by owner, and give
CatalogContext an openEvent operation plus createEvent, taking either a
CreateEventRequest or its fields spelled out. createEvent defaults to
a "Not implemented" error so existing implementations keep compiling
until they support it.

Let GuessMarketContext.placeOrder take an explicit side, keeping the
old signature as a default method that places a BUY so current callers
are unaffected.

Add OrderBookDTO and BookOrderDTO to describe per-option books with
best bid, best ask and spread, and ParticipantDTO for a user's
position, fees and P&L in an event. Carry the order books and
participants on EventDetailDTO and the market maker's name on
EventSummaryDTO.

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