Region pages: read the custom-price billing period off the API

[v0l]

Closes #95. Depends on api#302 (LNVPS/api#347, not merged yet) — don't merge before that lands, VmCustomPrice doesn't carry interval_amount/interval_type until it does.

Every "from" price on the region pages assumed monthly because the price endpoint had nothing else to say. Now it does, so:

• src/api.ts — VmCustomPrice gains interval_amount/interval_type.
• src/utils/vps-seo.ts regionOfferJsonLd — UnitPriceSpecification reads opts.price.interval_amount/interval_type instead of a hardcoded CostPlanIntervalType.MONTH, same as vpsTemplateJsonLd already does for standard plans.
• src/components/region-landing.tsx, src/pages/bitcoin-node-hosting.tsx — "From {price} per month, excluding VAT." → "From {price}, excluding VAT."; CostAmount already appends the interval itself once interval_type is on the price it's given.
• src/pages/vps-london.tsx, src/pages/vps-ireland.tsx, src/pages/vps-canada.tsx, bitcoin-node-hosting.tsx meta descriptions — new formatPriceWithInterval (src/utils/currency.ts) appends /<interval> to the plain-text price for the two Seo slots that can't take JSX.
• src/components/cost.tsx IntervalSuffix now delegates to the same formatIntervalText helper formatPriceWithInterval uses, so the day/month/year wording can't drift between the JSX and plain-text renderings.
• Tests: regionOfferJsonLd (billing period from the price, not hardcoded) and new currency.test.ts for the two helpers. bun test 41 pass, tsc --noEmit clean, build clean at a15c5ef.

Aside, not this PR: translating the changed strings surfaced pre-existing placeholder-adjacent punctuation gluing in a few non-Latin locales (e.g. German/Arabic/Japanese/Russian dropping the space after {diskType}) — already present in the currently-shipped translations for the same sentences under their old wording, not something this PR introduced. Flagging for whoever next touches the ollama_intl prompt template.