Refactor economics to expense-record ledger with correct Bubble cost

Replace pre-aggregated costs.json with expense_records.json (48 line-item
records) and payment_records.json. All monthly and cumulative totals are
computed deterministically in observatory/ledger.py. Correct Bubble.io to
$32/mo (since Feb 2025) — infrastructure €69.44/mo not €72.20.

projects/coulomb-pricing/observatory/models.py

@@ -4,7 +4,7 @@ from dataclasses import dataclass
 from decimal import Decimal
 from typing import Literal
 
-CostCategory = Literal["fixed", "variable"]
+ExpenseClass = Literal["infrastructure", "payment_processing"]
 MemberStatus = Literal["active", "churned", "paused"]
 PricingModelStatus = Literal["active", "candidate", "retired"]
 LiquidityStatus = Literal["burning", "neutral", "generating"]
@@ -33,14 +33,28 @@ class PricingModel:
 
 
 @dataclass(frozen=True)
-class CostEntry:
+class ExpenseRecord:
     id: str
-    name: str
-    category: CostCategory
+    period: str
+    vendor: str
+    description: str
+    cost_class: ExpenseClass
     amount: Decimal
     currency: str
-    cadence: str
-    allocation: str
+    source: str
+
+
+@dataclass(frozen=True)
+class PaymentRecord:
+    id: str
+    period: str
+    gross_amount: Decimal
+    fees_amount: Decimal
+    refunds_amount: Decimal
+    net_amount: Decimal
+    currency: str
+    source: str
+    member_count: int = 0
 
 
 @dataclass(frozen=True)
@@ -63,18 +77,6 @@ class Budget:
     started: str
 
 
-@dataclass(frozen=True)
-class RevenueEntry:
-    id: str
-    period: str
-    gross_amount: Decimal
-    fees_amount: Decimal
-    refunds_amount: Decimal
-    net_amount: Decimal
-    currency: str
-    source: str
-
-
 @dataclass(frozen=True)
 class MembershipRecord:
     id: str