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.

lnwire/channel_update_2.go

@@ -380,9 +380,23 @@ func (c ChanUpdateDisableFlags) IsEnabled() bool {
 	return c == 0
 }
 
-// String returns the bitfield flags as a string.
+// String returns a human-readable representation of the disable flags.
 func (c ChanUpdateDisableFlags) String() string {
-	return fmt.Sprintf("%08b", c)
+	if c.IsEnabled() {
+		return "Enabled"
+	}
+
+	incoming := c.IncomingDisabled()
+	outgoing := c.OutgoingDisabled()
+
+	switch {
+	case incoming && outgoing:
+		return "Disabled(incoming&outgoing)"
+	case incoming:
+		return "Disabled(incoming)"
+	default:
+		return "Disabled(outgoing)"
+	}
 }
 
 // Record returns the tlv record for the disable flags.