feat(payment_terms): Snapshot payment_term on invoice level

[D1353L]

Context

Before this change, invoice due dates were computed from customer.applicable_net_payment_term at every refresh. A later term change on the customer or the billing entity could silently move the due date of open drafts. To support structured terms (end_of_month, day_of_month, due_on_receipt), each invoice must keep the exact term that was used when it was created.

Description

• On invoice creation, the payment term is resolved once and stored on the invoice: the payment_term jsonb, the payment_term_source (customer or billing entity), and the integer net_payment_term alias.
• payment_due_date is computed from the snapshotted term.
• All refresh paths (draft refresh, VIES finalization, provider taxes, regeneration from voided, issuing date updates) read the invoice's own snapshot.
• Behavior change "Freeze on draft invoices": A term change on a customer or a billing entity no longer rewrites open drafts. It applies to future invoices only. The UpdateInvoicePaymentDueDateService classes are removed.
• Existing invoices have no jsonb snapshot. For them, Invoice#snapshotted_payment_term builds a net term from their own integer alias.
• Final nil relaxing for BillingEntity#net_payment_term on the model level.

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[lago-claude-ai-agent]

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

HOLD — invoices can snapshot the wrong billing entity's payment term.

• Resolve the fallback term from the invoice's effective billing entity in CreateGeneratingService and PreviewService; both select an alternate entity for the invoice but currently resolve through customer.billing_entity, producing the wrong snapshot, source, and due date when no customer term exists.
• Add regression coverage for generated and preview invoices using a subscription-specific or explicitly selected alternate billing entity.

[lago-claude-ai-agent]

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

HOLD — invoice generation and previews can snapshot a payment term from the wrong billing entity.

• Resolve the fallback term against the effective invoice billing entity in both CreateGeneratingService and PreviewService; they select an explicit/subscription billing entity but currently resolve from customer.billing_entity, producing the wrong term and due date in multi-entity flows. Add regression coverage where the two entities have different terms.