* fix(ari): pre-register ext channel id and defer bridge to its StasisStart
Two race conditions in the inbound ARI flow could leave a call silent:
1. Bridging both channels immediately after creating the ext media leg
raced against the ext channel entering the Stasis application; slow
chan_websocket handshakes produced "Channel not in Stasis application"
422 errors on addChannel.
2. Asterisk could fire StasisStart for the ext channel before the
externalMedia POST response returned, so _is_ext_channel returned
False and the event was dropped as an unknown outbound call.
Fixes:
- Generate the ext channel id as dograh-ext-<uuid> client-side and pass
it to Asterisk via the channelId query param. Mark the ext channel,
set its channel->run mapping, register the pending bridge entry, and
persist gathered_context.ext_channel_id all before the POST.
- Defer the bridge to a new _complete_bridge_after_ext_ready handler
triggered by the ext channel's own StasisStart. Both channels are
guaranteed in Stasis by then, so addChannel cannot 422.
- On POST failure or channelId mismatch, roll back the pending entry
and ERROR loudly.
* fix: replace in-memory dict with redis storage
