graph/db: add gossip version parameter to ForEachChannel

Update ForEachChannel to accept a gossip version parameter, allowing
callers to specify which gossip version's channels should be iterated.
This change prepares the graph database for supporting multiple gossip
versions while maintaining backward compatibility.

The KVStore implementation validates that only GossipVersion1 is
requested, returning ErrVersionNotSupportedForKVDB for other versions.
The SQLStore implementation validates known versions and passes the
version through to the underlying paginated query.

Update VersionedGraph to include a ForEachChannel method that
automatically uses its configured gossip version, and update the
DescribeGraph RPC handler to use the v1Graph instead of the global
graphDB.

graph/db/kv_store.go

@@ -411,10 +411,14 @@ func (c *KVStore) AddrsForNode(ctx context.Context, v lnwire.GossipVersion,
 // NOTE: If an edge can't be found, or wasn't advertised, then a nil pointer
 // for that particular channel edge routing policy will be passed into the
 // callback.
-func (c *KVStore) ForEachChannel(_ context.Context,
+func (c *KVStore) ForEachChannel(_ context.Context, v lnwire.GossipVersion,
 	cb func(*models.ChannelEdgeInfo, *models.ChannelEdgePolicy,
 		*models.ChannelEdgePolicy) error, reset func()) error {
 
+	if v != lnwire.GossipVersion1 {
+		return ErrVersionNotSupportedForKVDB
+	}
+
 	return forEachChannel(c.db, cb, reset)
 }