Curation desk ingest: forward the wave_nsfw event type #112

The curation desk ingest route forwards four event types (post, vote, curator_vote, flag) and answers 400 for anything else. The backend is gaining a fifth, wave_nsfw, which carries an image check's verdict for a wave so that waves with explicit pictures can be kept out of the shared waves feeds.

The gateway's type list has to include it, or the event is refused here before it reaches the backend, and the sender treats a 400 as permanent.

No other change: the payload travels as it is and the backend range-checks its fields, like the other types.