botserv: ASSIGN / UNASSIGN a bot to a channel

A channel gains a typed assigned_bot (event-logged, snapshotted). ASSIGN
<#channel> <bot> / UNASSIGN <#channel> is founder-gated (or admin oper). The
engine reconcile now also diffs channel assignments against tracked bot
memberships: a live bot IJOINs its assigned channels and PARTs unassigned ones
(new ServiceJoin/ServicePart actions), at burst and after commands; a deleted
bot's memberships are dropped. Engine test for join-on-assign / part-on-unassign.

src/grpc.rs

@@ -139,6 +139,8 @@ fn to_wire(entry: &LogEntry) -> Option<ReplicationEvent> {
         | Event::ChannelTopicSet { .. }
         | Event::ChannelSuspended { .. }
         | Event::ChannelUnsuspended { .. }
+        | Event::ChannelBotAssigned { .. }
+        | Event::ChannelBotUnassigned { .. }
         | Event::BotAdded(_)
         | Event::BotRemoved { .. } => return None,
     };