multi: update models.ChannelAuthProof with v2 field

Also update it to more closely match the persisted version which has the
v1 and v2 only fields as optional.

Refactor ChannelAuthProof to support both v1 and v2 channel
announcements:

- Add Version field to distinguish v1 from v2 proofs
- Wrap v1-specific fields (NodeSig1/2, BitcoinSig1/2) in fn.Option
  since v2 doesn't use them
- Add optional Signature field for v2's single schnorr signature
- Add constructor functions NewV1ChannelAuthProof and
  NewV2ChannelAuthProof to enforce correct initialization
- Add getter methods (NodeSig1(), BitcoinSig1(), etc.) that safely
  unwrap options, returning empty slices when not present

The IsEmpty() check is updated to handle both versions correctly.
Both stores validate v1-only for now.

graph/db/sql_store.go

@@ -2966,6 +2966,12 @@ func (s *SQLStore) DisconnectBlockAtHeight(height uint32) (
 func (s *SQLStore) AddEdgeProof(scid lnwire.ShortChannelID,
 	proof *models.ChannelAuthProof) error {
 
+	// For now, we only support v1 channel proofs.
+	if proof.Version != lnwire.GossipVersion1 {
+		return fmt.Errorf("only v1 channel proofs supported, got v%d",
+			proof.Version)
+	}
+
 	var (
 		ctx       = context.TODO()
 		scidBytes = channelIDToBytes(scid.ToUint64())
@@ -2975,10 +2981,10 @@ func (s *SQLStore) AddEdgeProof(scid lnwire.ShortChannelID,
 		res, err := db.AddV1ChannelProof(
 			ctx, sqlc.AddV1ChannelProofParams{
 				Scid:              scidBytes,
-				Node1Signature:    proof.NodeSig1Bytes,
-				Node2Signature:    proof.NodeSig2Bytes,
-				Bitcoin1Signature: proof.BitcoinSig1Bytes,
-				Bitcoin2Signature: proof.BitcoinSig2Bytes,
+				Node1Signature:    proof.NodeSig1(),
+				Node2Signature:    proof.NodeSig2(),
+				Bitcoin1Signature: proof.BitcoinSig1(),
+				Bitcoin2Signature: proof.BitcoinSig2(),
 			},
 		)
 		if err != nil {
@@ -4284,10 +4290,10 @@ func insertChannel(ctx context.Context, db SQLQueries,
 	if edge.AuthProof != nil {
 		proof := edge.AuthProof
 
-		createParams.Node1Signature = proof.NodeSig1Bytes
-		createParams.Node2Signature = proof.NodeSig2Bytes
-		createParams.Bitcoin1Signature = proof.BitcoinSig1Bytes
-		createParams.Bitcoin2Signature = proof.BitcoinSig2Bytes
+		createParams.Node1Signature = proof.NodeSig1()
+		createParams.Node2Signature = proof.NodeSig2()
+		createParams.Bitcoin1Signature = proof.BitcoinSig1()
+		createParams.Bitcoin2Signature = proof.BitcoinSig2()
 	}
 
 	// Insert the new channel record.
@@ -4483,12 +4489,16 @@ func buildEdgeInfoWithBatchData(chain chainhash.Hash,
 	// safely check if one signature is present to determine if we have the
 	// rest of the signatures for the auth proof.
 	if len(dbChan.Bitcoin1Signature) > 0 {
-		channel.AuthProof = &models.ChannelAuthProof{
-			NodeSig1Bytes:    dbChan.Node1Signature,
-			NodeSig2Bytes:    dbChan.Node2Signature,
-			BitcoinSig1Bytes: dbChan.Bitcoin1Signature,
-			BitcoinSig2Bytes: dbChan.Bitcoin2Signature,
+		// For v1 channels, we have four signatures.
+		if dbChan.Version == int16(lnwire.GossipVersion1) {
+			channel.AuthProof = models.NewV1ChannelAuthProof(
+				dbChan.Node1Signature,
+				dbChan.Node2Signature,
+				dbChan.Bitcoin1Signature,
+				dbChan.Bitcoin2Signature,
+			)
 		}
+		// TODO(elle): Add v2 support when needed.
 	}
 
 	return channel, nil