feat(payment_terms): PDF and e-invoice updates

[D1353L]

Context

Invoice PDFs and E-invoice files should display the new payment_term label. The legacy payment term sentence should be removed from the dunning email (payment request).

Labels:

term_type	🇬🇧 en
due_on_receipt	Due on receipt
net	Payment due within {N} days
end_of_month	Due at end of month
net_end_of_month	Due {N} days after end of month
days_end_of_month	Due {N} days, end of month
day_of_month	Due on the {D} of the following month

Day-of-month labels are offset-dependent:

month_offset	🇬🇧 en
0	Due on the {D} of this month
1	Due on the {D} of the following month
N ≥ 2	Due on the {D}, {N} months after invoice

Description

• Added a helper for building payment term labels.
• Updated PDF templates and E-invoice serializer to use the new helper.
• Legacy payment term sentence removed from the payment_request email.
• Added i18n locales.

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[lago-claude-ai-agent]

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

HOLD — the new shared labels can be grammatically wrong or contradict the invoice due date.

• Day-based terms are not pluralized, so days: 1 renders strings such as “Payment due within 1 days” in English and other pluralizing locales. Use locale plural forms and add singular coverage.
• A day_of_month term with month_offset: 0 rolls a past configured day into the following month, but #label always says “this month.” Make the wording accurate for the roll-forward case and test it against the calculated due date.

[lago-claude-ai-agent]

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

HOLD — day_of_month descriptions can disagree with the invoice's actual due date.

• PaymentTerm#label chooses the configured day and “this/following month” from month_offset alone, but due_date_for rolls a past offset-0 day into the next month and clamps invalid dates. For example, an invoice issued July 15 with day 10/offset 0 is due August 10 but renders “Due on the 10 of this month”; day 31 can likewise render for February while the due date is February 28/29. Derive wording from the effective due date (or use wording that remains accurate) and cover roll-forward and clamping cases.