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

sms.py

@@ -77,8 +77,9 @@ class SMSPoolProvider(NumberProvider):
         return self._get("service/retrieve_all", {})
 
     def provision_number(self, service, country, expiry_min):
-        return self._get("purchase/sms", {"service": service, "country": country,
-                                          "expiry": int(expiry_min)})
+        # `expiry` is passed for API compatibility but SMSPool ignores it —
+        # every rental hard-caps at 20 min (see MAX_RENTAL_MIN).
+        return self._get("purchase/sms", {"service": service, "country": country})
 
     def check_sms(self, order_id):
         return self._get("sms/check", {"orderid": order_id})
@@ -132,7 +133,9 @@ DEFAULT_PRICING = [
     {"duration_min": 20, "price_sats": 3000},
 ]
 
-MAX_RENTAL_MIN = 20  # SMSPool rental window ceiling; longer durations re-rent
+MAX_RENTAL_MIN = 20  # VERIFIED 2026-09-20: SMSPool ignores `expiry` param — rental numbers
+                     # hard-cap at 1200s (20 min) regardless of what's requested. Longer
+                     # sessions require a re-rent loop (not yet implemented).
 
 # --------------------------------------------------------------------------
 # Schema