channels: drop stale membership caches on join/leave

After channels.leaveChannel, a client that polls channels.getFullChannel kept
receiving a projection that still showed it as an active member (left=false)
until the per-(viewer,channel) RPC projection cache and the store-level member
cache lapsed on their own or the async read-model NOTIFY landed. The client
therefore kept an open compose box while every send was already rejected with
CHANNEL_PRIVATE - most visible on public forum supergroups, where getFullChannel
keeps succeeding via the preview path instead of tearing the chat down.

Every other membership-mutating path already busts these caches synchronously;
join/leave/invite/request-approval did not. Add:

- store: invalidateChannelMembershipCaches (row + member + dialog caches),
  called post-commit from JoinChannel, LeaveChannel, ImportInvite,
  InviteToChannel.
- rpc: invalidateChannelMembershipProjection (channelFullProjectionCache pair),
  called from the join/leave/invite/hide-requests handlers for every user whose
  membership changed.

internal/store/postgres/channel_invite_import.go

@@ -103,6 +103,7 @@ func (s *ChannelStore) ImportInvite(ctx context.Context, req domain.ImportChanne
 		return domain.CreateChannelResult{}, fmt.Errorf("commit import channel invite: %w", err)
 	}
 	committed = true
+	s.invalidateChannelMembershipCaches(result.Channel.ID, req.UserID)
 	recipients, _ := s.ListActiveChannelMemberIDs(ctx, req.UserID, result.Channel.ID, 0)
 	result.Recipients = recipients
 	return result, nil