Newsletter opt-in: silent Sendy subscribe on join

Pre-checked "InstantAdPay newsletter" opt-in on the join screen (read by both the
email-code and password signup paths). On new-account creation only, the server
silently subscribes them to the Sendy "InstantAdPay Newsletter" list
(boolean=true, opt-out always wins). New sendy.js helper reads the API key from
SENDY_API_KEY env or DATA_DIR/sendy.key on the volume (same pattern as
sendgrid.key); subscribe is fire-and-forget and never blocks signup.

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

public/my.html

@@ -18,6 +18,10 @@
       <p class="lead" id="introLead" style="font-size:16px">Join free with your email. Your wallet only
         comes out when you buy a package or switch on payouts, and it stays yours the whole time.</p>
     </section>
+    <label id="nlOptinRow" style="display:flex;gap:9px;align-items:flex-start;margin:0 4px 16px;font-size:13px;color:var(--muted,#8ba69c);cursor:pointer">
+      <input type="checkbox" id="nlOptin" checked style="margin-top:2px;flex:0 0 auto">
+      <span>Keep me posted with the InstantAdPay newsletter (tips and updates). You can unsubscribe anytime.</span>
+    </label>
     <div class="card" id="sponsorNote" style="border-color:var(--mint);text-align:center" hidden>
       <p style="margin:0"><span class="muted small">You're joining the line of</span><br>
         <b id="sponsorNoteName" style="font-size:19px;color:var(--mint)"></b></p>
@@ -687,7 +691,7 @@
 </div>
 <script src="/assets/common.js?v=20260908c"></script>
 <script src="/assets/wallet.js?v=20260907q"></script>
-<script src="/assets/my.js?v=20260908f"></script>
+<script src="/assets/my.js?v=20260908g"></script>
 <script src="/assets/chat.js?v=20260907l"></script>
 </body>
 </html>