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 %}