Clarify 20-min rental ceiling (verified: SMSPool ignores expiry param)

- sms.py: drop misleading expiry param from /purchase/sms (SMSPool hard-caps
  rentals at 1200s/20min regardless); document MAX_RENTAL_MIN as verified
- surface the ceiling in customer UI + admin panel + feasibility doc

templates/sms.html

@@ -6,6 +6,7 @@
     <h1>Temporary <b>SMS</b></h1>
     <p class="sub">Rent a real number for a few minutes to receive a verification code. No outbound. Pay with your wallet balance.</p>
     <p class="disclaimer">Availability depends on the service you're verifying with. Some services reject temporary, VoIP, or recycled numbers.</p>
+    <p class="disclaimer">Rentals are capped at <b>20 minutes</b> (provider limit).</p>
   </header>
 
   {% if session %}

templates/sms_admin.html

@@ -3,7 +3,7 @@
 {% block body %}
 <div style="max-width:900px;margin:0 auto;padding:24px 0">
   <h1 style="font-size:1.9rem;letter-spacing:-.5px">SMS <b style="color:#0ff">Admin</b></h1>
-  <p style="color:#94a3b8;margin:6px 0 16px">SMSPool balance: <b style="color:#0ff">{{ smspool_balance if smspool_balance is not none else 'unavailable' }} USD</b></p>
+  <p style="color:#94a3b8;margin:6px 0 16px">SMSPool balance: <b style="color:#0ff">{{ smspool_balance if smspool_balance is not none else 'unavailable' }} USD</b> · <span style="color:#f59e0b">Rental ceiling: 20 min (SMSPool hard cap — longer sessions need a re-rent loop)</span></p>
 
   <div style="display:grid;grid-template-columns:repeat(auto-fit,minmax(140px,1fr));gap:12px;margin-bottom:20px">
     <div style="background:rgba(20,28,40,.6);border:1px solid rgba(255,255,255,.08);border-radius:12px;padding:14px"><div style="font-size:.7rem;color:#94a3b8;letter-spacing:.1em">ACTIVE</div><div style="font-size:1.6rem;color:#fff">{{ stats.active }}</div></div>