feat(customcurrency): support custom currency subscriptions

[GAlexIHU]

cc @chrisgacsal @turip — this is the semantic cutover in a three-PR stack.

stack and rollout

1. feat(productcatalog): support custom currencies #4738 adds custom-currency identities, persistence, and compatibility rules to plans, rate cards, add-ons, and assignments.
2. feat(customcurrency): materialize subscription item currencies #4742 expands subscription-item persistence and materializes currency for every new or rewritten priced item, while legacy null-currency items stay readable. It deliberately changes no subscription semantics.
3. This PR performs the cutover after that expand/write step: Subscription.InvoiceCurrency is the invoice FIAT; each priced subscription item carries its actual FIAT or managed custom currency.

Deployed environments should roll out #4742 first, backfill existing priced items, verify none remain without currency, then deploy this PR. OSS has the same requirement documented in the migration guide; this PR is intentionally breaking for upgrades that skip it.

product behavior

• customer and subscription invoice currencies remain FIAT and must align with any FIAT used by the plan/items
• custom-only plans use the customer FIAT as invoiceCurrency
• item currency is materialized and immutable across edits/add-on overlays
• credit_only subscriptions do not require a cost basis
• credit_then_invoice subscriptions require each custom → invoice-FIAT pair
• cost bases can remain dynamic or be pinned per currency pair; add-ons reuse existing pins or resolve a new pair at their effective time
• v3 exposes invoiceCurrency; v1 keeps its existing currency compatibility surface

Actual billing conversion is still outside this PR. Until it lands, automatic subscription sync skips custom-currency subscriptions and explicit sync fails before creating billing artifacts. Dynamic conversion will resolve at the charge FSP start; pinned subscriptions will use their stored pair.

[greptile-apps]

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