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/ledger.py

@@ -0,0 +1,121 @@
+from __future__ import annotations
+
+from decimal import Decimal, ROUND_HALF_UP
+
+from .models import (
+    Budget,
+    ExpenseRecord,
+    MembershipRecord,
+    MonthlyPlatformCost,
+    PaymentRecord,
+)
+
+TWOPLACES = Decimal("0.01")
+
+
+def _quantize(value: Decimal) -> Decimal:
+    return value.quantize(TWOPLACES, rounding=ROUND_HALF_UP)
+
+
+def convert_to_eur(amount: Decimal, currency: str, fx_rates: dict[str, Decimal]) -> Decimal:
+    if currency == "EUR":
+        return amount
+    if currency == "USD":
+        return amount * fx_rates.get("USD/EUR", Decimal("1"))
+    raise ValueError(f"unsupported expense currency: {currency}")
+
+
+def aggregate_infrastructure_by_period(
+    expenses: list[ExpenseRecord],
+    fx_rates: dict[str, Decimal],
+    reporting_currency: str = "EUR",
+) -> dict[str, Decimal]:
+    if reporting_currency != "EUR":
+        raise ValueError("only EUR reporting currency is supported")
+
+    totals: dict[str, Decimal] = {}
+    for record in expenses:
+        if record.cost_class != "infrastructure":
+            continue
+        totals[record.period] = totals.get(record.period, Decimal("0")) + convert_to_eur(
+            record.amount, record.currency, fx_rates
+        )
+    return {period: _quantize(total) for period, total in totals.items()}
+
+
+def payment_processing_by_period(payments: list[PaymentRecord]) -> dict[str, Decimal]:
+    totals: dict[str, Decimal] = {}
+    for record in payments:
+        totals[record.period] = totals.get(record.period, Decimal("0")) + record.fees_amount
+    return {period: _quantize(total) for period, total in totals.items()}
+
+
+def _period_sort_key(period: str) -> tuple[int, int]:
+    year, month = period.split("-")
+    return int(year), int(month)
+
+
+def periods_from_budget_through_latest(
+    budget: Budget,
+    expenses: list[ExpenseRecord],
+    payments: list[PaymentRecord],
+) -> list[str]:
+    candidates = {budget.started}
+    candidates.update(record.period for record in expenses)
+    candidates.update(record.period for record in payments)
+    latest = max(candidates, key=_period_sort_key)
+    periods: list[str] = []
+    year, month = map(int, budget.started.split("-"))
+    end_year, end_month = map(int, latest.split("-"))
+    while (year, month) <= (end_year, end_month):
+        periods.append(f"{year}-{month:02d}")
+        month += 1
+        if month > 12:
+            month = 1
+            year += 1
+    return periods
+
+
+def active_members_for_period(period: str, members: list[MembershipRecord]) -> int:
+    period_start = f"{period}-01"
+    count = 0
+    for member in members:
+        if member.joined_at[:7] > period:
+            continue
+        if member.churned_at and member.churned_at[:7] < period:
+            continue
+        if member.status == "active" or (
+            member.churned_at and member.churned_at[:7] >= period
+        ):
+            count += 1
+    return count
+
+
+def build_monthly_ledger(
+    budget: Budget,
+    expenses: list[ExpenseRecord],
+    payments: list[PaymentRecord],
+    members: list[MembershipRecord],
+    fx_rates: dict[str, Decimal],
+) -> list[MonthlyPlatformCost]:
+    infrastructure = aggregate_infrastructure_by_period(expenses, fx_rates)
+    processing = payment_processing_by_period(payments)
+    payment_by_period = {record.period: record for record in payments}
+    rows: list[MonthlyPlatformCost] = []
+
+    for period in periods_from_budget_through_latest(budget, expenses, payments):
+        payment = payment_by_period.get(period)
+        rows.append(
+            MonthlyPlatformCost(
+                period=period,
+                infrastructure_cost=infrastructure.get(period, Decimal("0.00")),
+                payment_processing_cost=processing.get(period, Decimal("0.00")),
+                active_members=(
+                    payment.member_count
+                    if payment and payment.member_count
+                    else active_members_for_period(period, members)
+                ),
+                gross_revenue=payment.gross_amount if payment else Decimal("0.00"),
+            )
+        )
+    return rows