refactor: centralize constants, eliminate unwraps, remove dead code, resolve TODOs

- R13+R16: Replace .expect() with .context()? in main.rs and identity.rs
- R17+R18+R19: Fix unwrap() calls in helpers and js-engine
- R20+R21: Remove #[allow(dead_code)] annotations and delete truly dead code
- R22-R26: Create constants.rs module, replace 21 hardcoded values across 12 files
- R28+R29: LND/DWN timeouts already present — verified
- R30-R33: Remove TODO comments, implement marketplace payment check

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

core/archipelago/src/electrs_status.rs

@@ -8,7 +8,6 @@ use tokio::net::TcpStream;
 
 const ELECTRUMX_HOST: &str = "127.0.0.1";
 const ELECTRUMX_PORT: u16 = 50001;
-const BITCOIN_RPC_URL: &str = "http://127.0.0.1:8332/";
 const ELECTRUMX_DATA_DIR: &str = "/var/lib/archipelago/electrumx";
 // Approximate final index size in bytes for mainnet (~130GB for ElectrumX full index as of 2026)
 const ESTIMATED_FULL_INDEX_BYTES: f64 = 130_000_000_000.0;
@@ -132,7 +131,7 @@ async fn bitcoin_network_height() -> Result<u64> {
         "params": []
     });
     let resp = client
-        .post(BITCOIN_RPC_URL)
+        .post(crate::constants::BITCOIN_RPC_URL)
         .header("Content-Type", "application/json")
         .header("Authorization", bitcoin_rpc_auth().await)
         .body(body.to_string())