feat(payment_terms): Backfill customers and billing_entities

[D1353L]

Context

The new payment_term field needs to be backfilled for existing customers and billing entities. Equivalent:
Legacy net_payment_terms: N days -> { term_type: "net", days: N }

Description

• Migration for backfilling customers and billing_entities tables.
• As the customers table is large, updates are processed in batches of 10k records.
• MigrationCustomer is used as an ActiveRecord stub to avoid "Undeclared attribute type for enum" errors.

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[lago-claude-ai-agent]

Automated pre-review (advisory, not a required check) — verdict: HOLD · CI green

HOLD — The new data migration has no test coverage.

• Add a migration spec covering both customers and billing entities, including backfilling legacy values while preserving existing payment_term values and customer rows whose legacy value is null.

[lago-claude-ai-agent]

Automated pre-review (advisory, not a required check) — verdict: HOLD · CI green

HOLD — the backfill matches the structured net representation and preserves existing JSONB values, but the migration has no behavioral coverage.

• Add an executable migration spec covering both customers and billing entities, including null legacy terms and pre-existing payment_term values, so the backfill and skip conditions are protected.