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.

discovery/gossiper.go

@@ -3423,6 +3423,7 @@ func (d *AuthenticatedGossiper) handleChanUpdate(ctx context.Context,
 	// signs a different SCID than the database SCID, but since there will
 	// only be a difference if AuthProof == nil, this is fine.
 	update := &models.ChannelEdgePolicy{
+		Version:                   upd.GossipVersion(),
 		SigBytes:                  upd.Signature.ToSignatureBytes(),
 		ChannelID:                 chanInfo.ChannelID,
 		LastUpdate:                timestamp,