Expose company_id on subscription responses for per-company VAT display

[v0l]

Problem

The customer subscription responses (GET /api/v1/subscriptions, GET /api/v1/subscriptions/{id}) don't carry a seller company_id. ApiSubscription and ApiSubscriptionLineItem both omit it, even though the underlying Subscription row already has company_id (lnvps_db/src/model.rs).

Why it matters

VAT rates are per seller company: account.tax[] is keyed by company_id, and elsewhere clients match it against a company id to pick the right rate (e.g. VmHostRegion.company_id is documented as "match against account.tax[].company_id for the applicable VAT rate").

The web app just added an "incl. tax" price toggle. For VMs it resolves the correct rate via region.company_id, but subscriptions (IP ranges, DNS, …) expose no company id, so the UI has to fall back to the primary (first) tax entry. That's correct for single-company deployments but wrong once a customer holds subscriptions across multiple companies with different VAT rates.

Proposal

Expose the seller company_id on the subscription response so clients can gross-up prices with the right rate. Either (or both):

interface SubscriptionLineItem {
  // ...
  company_id: number; // seller company for this line item's VAT rate
}

or, since a subscription is billed by a single company and the value already lives on the Subscription row, the simpler:

interface Subscription {
  // ...
  company_id: number;
}

Read-only, additive — no breaking change. The subscription-level field is the least duplication; the line-item field mirrors how VM pricing already carries company context. Either lets the client match account.tax[].company_id and drop the primary-rate fallback.

Related: the host CPU arch exposure (#210) followed the same "surface the id the client needs to resolve a per-account value" pattern.