Mainnet indexer fix + retire testnet copy

- chain.js: derive the chain tip as the MAX height across the RPC pool and scan
  getLogs against that same synced node. Load-balanced public RPCs (publicnode)
  intermittently answer eth_blockNumber from a replica lagging thousands of
  blocks behind, which stalled the mainnet scan (latest < lastBlock, 0 events)
  and could skip freshly-mined events. This unblocks the live ledger + the
  purchase-confirmation email (both driven by the event scan).
- disclaimer: replace the "rehearsal/testnet" section with a live-on-Polygon
  real-money notice (it's live now).
- Point the dead Amoy fallbacks (purchase email tx link, contract-page source
  link) at Polygon mainnet / the verified mainnet contract.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server.js

@@ -279,7 +279,7 @@ async function emailOnEvent(ev) {
   };
   if (ev.type === 'Purchase') {
     const cc = chain.getConfig();
-    const txUrl = (cc.explorer ? cc.explorer.replace(/\/+$/, '') : 'https://amoy.polygonscan.com') + '/tx/' + ev.tx;
+    const txUrl = (cc.explorer ? cc.explorer.replace(/\/+$/, '') : 'https://polygonscan.com') + '/tx/' + ev.tx;
     let bal = ev.creditAmount;
     try { bal = await chain.creditBalance(ev.buyerId, ev.creditType); } catch (e) {}
     await notify(ev.buyerId, 'Your InstantAdPay purchase is confirmed',