fix(exchange): make PurchaseHistoryRecord.MonthlyCost a *float64 to distinguish absent from zero

[cristim]

Problem

PurchaseHistoryRecord.MonthlyCost is float64 (not *float64), so a nil/unknown monthly cost is stored as 0. Per feedback_nullable_not_zero, an absent monthly cost coerced to 0 is indistinguishable from a real $0/mo commitment in any downstream sum or average.

File: internal/purchase/execution.go:601, 904-913

The derefFloat64 helper is honest about the coercion in its comment, but the table column type forces the information loss.

Compare: RecommendationRecord.MonthlyCost is already *float64 — this field should mirror that.

Fix

1. Change PurchaseHistoryRecord.MonthlyCost to *float64.
2. Add a nullable column migration for the history schema.
3. Update all readers/writers that currently dereference the field unconditionally.

Notes

• This is migration-bearing — batch with the next history-schema change.
• Out of scope for the immediate fix/exchange-fail-loud PR (L4 finding from docs/code-review/19-hardcoded-fallbacks-core.md).

[cristim]

Verification sweep against main (101f099fb) on 2026-07-27 finds this already resolved.
PurchaseHistoryRecord.MonthlyCost is now *float64 and migration 000063 dropped the NOT NULL constraint, so missing cost is representable as null rather than 0.
Evidence: commit 53c4900 (PR #848, under #255).
Recommending close.

[cristim]

Verified resolved at 3c0f8ac: PurchaseHistoryRecord.MonthlyCost is a *float64 backed by a nullable purchase_history.monthly_cost column, so an unknown monthly cost is stored as NULL rather than coerced to 0, and the exchange, scheduler and store writers pass the pointer through unchanged. Evidence: internal/config/types.go:809 and internal/database/postgres/migrations/000063_purchase_history_monthly_cost_nullable.up.sql (#848). Residual axes checked: internal/purchase/execution.go:907 and internal/scheduler/scheduler.go:1508 preserve nil; the analytics collector and validation nil-check before dereferencing; the derefFloat64 helper is gone. Closing as completed; reopen if the behaviour recurs.