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/localchans/manager.go

@@ -364,6 +364,7 @@ func (r *Manager) createEdge(channel *channeldb.OpenChannel,
 	// be updated with the new values in the call to processChan below.
 	timeLockDelta := uint16(r.DefaultRoutingPolicy.TimeLockDelta)
 	edge := &models.ChannelEdgePolicy{
+		Version:                   lnwire.GossipVersion1,
 		ChannelID:                 shortChanID.ToUint64(),
 		LastUpdate:                timestamp,
 		TimeLockDelta:             timeLockDelta,

routing/localchans/manager_test.go

@@ -64,6 +64,7 @@ func TestManager(t *testing.T) {
 	}
 
 	currentPolicy := models.ChannelEdgePolicy{
+		Version:      lnwire.GossipVersion1,
 		MinHTLC:      minHTLC,
 		MessageFlags: lnwire.ChanUpdateRequiredMaxHtlc,
 	}
@@ -451,6 +452,7 @@ func TestCreateEdgeLower(t *testing.T) {
 	require.NoError(t, err)
 
 	expectedEdge := &models.ChannelEdgePolicy{
+		Version:                   lnwire.GossipVersion1,
 		ChannelID:                 8,
 		LastUpdate:                timestamp,
 		TimeLockDelta:             7,
@@ -542,6 +544,7 @@ func TestCreateEdgeHigher(t *testing.T) {
 	require.NoError(t, err)
 
 	expectedEdge := &models.ChannelEdgePolicy{
+		Version:                   lnwire.GossipVersion1,
 		ChannelID:                 8,
 		LastUpdate:                timestamp,
 		TimeLockDelta:             7,