fix: correctly price unlimited traffic (0 GB) in classic subscription mode

_calculate_traffic_price treated unlimited traffic as free because
base_gb=0 triggered the `if base_gb > 0 else 0` guard, skipping
the price lookup. Added early return for total_gb==0 to use the
configured unlimited tier price.

app/services/pricing_engine.py

@@ -479,6 +479,11 @@ class PricingEngine:
         Prevents purchased top-ups from inflating the tier lookup."""
         total_gb = traffic_limit_gb or 0
         purchased_gb = purchased_traffic_gb or 0
+
+        # 0 = unlimited traffic — has its own price tier, return directly
+        if total_gb == 0:
+            return settings.get_traffic_price(0)
+
         base_gb = max(0, total_gb - purchased_gb)
 
         base_price = settings.get_traffic_price(base_gb) if base_gb > 0 else 0

tests/test_pricing_engine.py

@@ -232,12 +232,19 @@ class TestCalculateTrafficPrice:
             price = engine._calculate_traffic_price(traffic_limit_gb=125, purchased_traffic_gb=100)
         assert price == 11000  # NOT 12000
 
-    def test_zero_traffic(self):
+    def test_unlimited_traffic_has_price(self):
         engine = PricingEngine()
         with patch('app.services.pricing_engine.settings') as ms:
-            ms.get_traffic_price.return_value = 0
+            ms.get_traffic_price.side_effect = lambda gb: {0: 20000, 5: 2000}.get(gb, 0)
             price = engine._calculate_traffic_price(traffic_limit_gb=0, purchased_traffic_gb=0)
-        assert price == 0
+        assert price == 20000  # 0 GB = unlimited, charged at unlimited tier
+
+    def test_unlimited_traffic_ignores_purchased(self):
+        engine = PricingEngine()
+        with patch('app.services.pricing_engine.settings') as ms:
+            ms.get_traffic_price.side_effect = lambda gb: {0: 20000, 50: 5000}.get(gb, 0)
+            price = engine._calculate_traffic_price(traffic_limit_gb=0, purchased_traffic_gb=50)
+        assert price == 20000  # unlimited tier, purchased ignored
 
     def test_purchased_exceeds_total(self):
         engine = PricingEngine()