feat: resolve taxes for product fees

[endenis]

Context

Part of the "products and plans" project.

Product fees did not resolve taxes from rate cards.

Description

• Resolve taxes using Rate Card -> Plan -> Customer -> Billing Entity.
• Use the fee's billing entity for Product fees.
• Mark affected draft invoices for refresh when rate rard taxes change.
• Keep taxes on finalized invoice unchanged.

[lago-claude-ai-agent]

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

HOLD — Draft product invoices are not invalidated when an assigned rate-card tax itself is updated or deleted.

• Extend tax update/delete invalidation to draft invoices whose product fees reference rate cards using that tax; Tax#applicable_customers does not include rate-card assignments, so these invoices retain stale tax amounts or snapshots.
• Add focused regression coverage for updating and deleting a rate-card tax used by a draft product invoice.

[tiagolupepic]

it's not the scope of this PR but the previous applicable_taxes is a bit problematic already. We perform the ApplyTaxesService and accidentally does a N+1 https://github.com/getlago/lago-api/blob/main/app/services/invoices/compute_amounts_from_fees.rb#L20. I'd add a task in Linear to tackle this and avoid continue with the same issue for product_taxes.

[tiagolupepic]

question also for @rsempe @brunomiguelpinto do we need to apply the refresh for the rate_override as well?

[rsempe]

no extra refresh needed: overrides carry no taxes of their own (taxes are just on the card, right?)

[tiagolupepic]

I see, in this case what is the rule to apply taxes? I thought was because of the pricing in properties.

[rsempe]

Should we have to add validates :rate_card_rate, presence: true, if: :product? on Fee?