fix: ElectrumX status uses headers.subscribe (returns height correctly)

The previous blockchain.numblocks.subscribe call returned data in a
format the parser couldn't extract height from. headers.subscribe
returns {height: N, hex: "..."} which is properly parsed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/electrs_status.rs

@@ -72,8 +72,8 @@ fn electrumx_indexed_height() -> Result<u64> {
         .set_write_timeout(Some(Duration::from_secs(5)))
         .context("set_write_timeout")?;
 
-    // blockchain.numblocks.subscribe returns current block height directly
-    let req = r#"{"id":1,"method":"blockchain.numblocks.subscribe","params":[]}
+    // blockchain.headers.subscribe returns {"height": N, "hex": "..."}
+    let req = r#"{"id":1,"method":"blockchain.headers.subscribe","params":[]}
 "#;
     stream.write_all(req.as_bytes())?;
     stream.flush()?;