Fix cumulative platform cost Stripe double-counting

Split infrastructure vs payment-processing costs. Liquidity burn now
uses infrastructure cash out only (€1,155.20 cumulative) because Stripe
fees are already deducted from net member payments. Total platform cost
(€1,158.24) remains visible for gross-margin economics.

projects/coulomb-pricing/observatory/models.py

@@ -46,10 +46,15 @@ class CostEntry:
 @dataclass(frozen=True)
 class MonthlyPlatformCost:
     period: str
-    platform_cost: Decimal
+    infrastructure_cost: Decimal
+    payment_processing_cost: Decimal
     active_members: int
     gross_revenue: Decimal
 
+    @property
+    def total_platform_cost(self) -> Decimal:
+        return self.infrastructure_cost + self.payment_processing_cost
+
 
 @dataclass(frozen=True)
 class Budget:
@@ -85,7 +90,9 @@ class EconomicsSnapshot:
     currency: str
     active_members: int
     monthly_revenue: Decimal
-    monthly_platform_cost: Decimal
+    monthly_infrastructure_cost: Decimal
+    monthly_payment_processing_cost: Decimal
+    monthly_total_platform_cost: Decimal
     cost_per_member: Decimal
     gross_margin: Decimal
     gross_margin_pct: Decimal
@@ -101,7 +108,9 @@ class LiquiditySummary:
     through_period: str
     initial_budget: Decimal
     cumulative_member_payments: Decimal
-    cumulative_platform_cost: Decimal
+    cumulative_infrastructure_cost: Decimal
+    cumulative_payment_processing_cost: Decimal
+    cumulative_total_platform_cost: Decimal
     cumulative_net_liquidity: Decimal
     remaining_budget: Decimal
     liquidity_status: LiquidityStatus