MoonPay card on-ramp for crypto-new members (zero custody)

- GET /api/public/moonpay-url: returns a MoonPay checkout link for POL on
  Polygon (pol_polygon). With partner keys in config (moonpayPublicKey/
  moonpaySecretKey, now PATCHable), the URL is HMAC-signed and prefilled
  with the member's own wallet address + shortfall amount; without keys it
  falls back to MoonPay's generic buy page. MoonPay is merchant of record -
  the site never touches funds.
- Join page: when the connected wallet can't cover the Premium entry, a
  funding box appears with the exact shortfall, a buy button (new tab), and
  a refresh-balance button.
- Training page: "buy POL with a card" callout after the funding video with
  the three things to get right (POL, Polygon network, own address).
- Start page: one-line card-buy pointer under the sponsor card.
- Chatbot canned answer + AI system prompt updated (funding guidance).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

public/chat.js

@@ -21,7 +21,7 @@
     {k:['metamask','wallet','create wallet','install','set up wallet','setup wallet'],
      a:()=>`Install MetaMask only from the <a href="https://metamask.io/" target="_blank" rel="noopener noreferrer">official site</a> or your device's official app store, then create your wallet. <a href="/training">Video 2 in the training</a> walks through it step by step. Never share your Secret Recovery Phrase with anyone — not the team, not this site, not a "support agent".`},
     {k:['fund','funding','buy pol','get pol','deposit','exchange','transfer','send pol'],
-     a:()=>`You'll need about ${pol()} POL plus a little extra for gas, on Polygon Mainnet. <a href="/training">Video 3 in the training</a> shows how to fund your wallet. Double-check you're sending POL on the Polygon network — not another chain.`},
+     a:()=>`You'll need about ${pol()} POL plus a little extra for gas, on Polygon Mainnet. <a href="/training">Video 3 in the training</a> shows how to fund your wallet. Brand new to crypto? You can buy POL directly with a debit/credit card, Apple Pay, or Google Pay via <a href="https://www.moonpay.com/buy/pol" target="_blank" rel="noopener noreferrer">MoonPay</a> — pick POL on the Polygon network and send it to your own wallet address. The join page also offers this automatically if your connected wallet is short. Double-check you're getting POL on Polygon — not another chain.`},
     {k:['join','buy','purchase','premium position','enter','sign up','signup','register','get started','start'],
      a:()=>`Ready to join? Go to the <a href="/start">Getting Started page</a> — it shows the current team sponsor and lets you connect your wallet and buy your position right on the site, with the sponsor ID filled in automatically. <a href="/training">Video 4 in the training</a> walks through the whole process (and Video 5 covers the dApp backup method).`},
     {k:['sponsor','current sponsor','who do i join under','placement','whose link','referral'],