graph/db: add v2 fields to ChannelEdgePolicy model

Extend ChannelEdgePolicy to support v2 channel updates by adding:

- Version field to track gossip protocol version (v1 or v2).
- LastBlockHeight for v2's block-height-based timestamps.
- SecondPeer flag to indicate which peer announced the policy in v2.
- DisableFlags for v2-specific channel disable signaling.
- ExtraSignedFields map for v2 extra signed TLV data.

Add version-aware methods:
- IsNode1() determines if the policy was announced by node_1, handling
  both v1 (via ChannelFlags direction bit) and v2 (via SecondPeer).
- IsDisabled() checks disable status using ChannelFlags for v1 and
  DisableFlags for v2.
- String() provides version-appropriate string representations.

The new fields use zero values for v1 compatibility (Version defaults
to GossipVersion1, LastBlockHeight to 0, SecondPeer to false). This
lays the groundwork for v2 policy support; a subsequent commit will
handle reading and writing these fields from/to the database.

routing/pathfind_test.go

@@ -389,6 +389,7 @@ func parseTestGraph(t *testing.T, useCache bool, path string) (
 		}
 
 		edgePolicy := &models.ChannelEdgePolicy{
+			Version:                   lnwire.GossipVersion1,
 			SigBytes:                  testSig.Serialize(),
 			MessageFlags:              lnwire.ChanUpdateMsgFlags(edge.MessageFlags),
 			ChannelFlags:              channelFlags,
@@ -740,7 +741,9 @@ func createTestGraphFromChannels(t *testing.T, useCache bool,
 				channelFlags |= lnwire.ChanUpdateDisabled
 			}
 
+			//nolint:ll
 			edgePolicy := &models.ChannelEdgePolicy{
+				Version:                   lnwire.GossipVersion1,
 				SigBytes:                  testSig.Serialize(),
 				MessageFlags:              msgFlags,
 				ChannelFlags:              channelFlags,
@@ -772,7 +775,9 @@ func createTestGraphFromChannels(t *testing.T, useCache bool,
 			}
 			channelFlags |= lnwire.ChanUpdateDirection
 
+			//nolint:ll
 			edgePolicy := &models.ChannelEdgePolicy{
+				Version:                   lnwire.GossipVersion1,
 				SigBytes:                  testSig.Serialize(),
 				MessageFlags:              msgFlags,
 				ChannelFlags:              channelFlags,