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>

private/join-now.html

@@ -27,6 +27,13 @@
       <div class="dj-kv"><span>Balance</span><b id="bal">—</b></div>
     </div>
     <div id="already" style="display:none;margin-top:12px;color:#f0b4aa;font-size:14px"></div>
+    <div id="fundBox" style="display:none;margin-top:14px;padding:14px;border:1px solid rgba(240,197,109,.4);border-radius:12px;background:rgba(240,197,109,.06)">
+      <div style="font-weight:800;margin-bottom:6px">💳 Need POL? Buy it with a card.</div>
+      <p id="fundMsg" style="color:var(--muted);font-size:13px;margin:0 0 10px"></p>
+      <button id="fundBtn" class="btn btn-secondary" style="width:100%">Buy POL with card / Apple Pay (MoonPay) →</button>
+      <button id="fundRefresh" class="btn btn-secondary btn-sm" style="width:100%;margin-top:8px">I've bought it — refresh my balance</button>
+      <p class="micro" style="margin:8px 0 0">MoonPay opens in a new tab. Choose <strong>POL on Polygon</strong>; the crypto is delivered to <strong>your own wallet</strong> — this site never touches your money. Card fees are MoonPay's (~4.5%), and first purchases can take a few minutes to arrive.</p>
+    </div>
   </div>
 
   <div id="joinBox" class="dj-card" style="display:none">