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.

netann/chan_status_manager_test.go

@@ -116,12 +116,14 @@ func createEdgePolicies(t *testing.T, channel *channeldb.OpenChannel,
 
 	return edgeInfo,
 		&models.ChannelEdgePolicy{
+			Version:      lnwire.GossipVersion1,
 			ChannelID:    channel.ShortChanID().ToUint64(),
 			ChannelFlags: dir1,
 			LastUpdate:   time.Now(),
 			SigBytes:     testSigBytes,
 		},
 		&models.ChannelEdgePolicy{
+			Version:      lnwire.GossipVersion1,
 			ChannelID:    channel.ShortChanID().ToUint64(),
 			ChannelFlags: dir2,
 			LastUpdate:   time.Now(),
@@ -222,6 +224,7 @@ func (g *mockGraph) ApplyChannelUpdate(update *lnwire.ChannelUpdate1,
 	timestamp := time.Unix(int64(update.Timestamp), 0)
 
 	policy := &models.ChannelEdgePolicy{
+		Version:      lnwire.GossipVersion1,
 		ChannelID:    update.ShortChannelID.ToUint64(),
 		ChannelFlags: update.ChannelFlags,
 		LastUpdate:   timestamp,