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.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/store/postgres/channel_invite_members.go

@@ -125,6 +125,7 @@ func (s *ChannelStore) InviteToChannel(ctx context.Context, channelID, inviterUs
 		return domain.CreateChannelResult{}, fmt.Errorf("commit invite channel: %w", err)
 	}
 	committed = true
+	s.invalidateChannelMembershipCaches(channelID, invitedIDs...)
 	recipients, _ := s.ListActiveChannelMemberIDs(ctx, inviterUserID, channelID, 0)
 	return domain.CreateChannelResult{Channel: channel, Members: members, Message: msg, Event: event, Recipients: recipients}, nil
 }