feat(customers): group credit notes by entity

[aquinofb]

Context

The customer detail page is replacing its single "Total amount available" credit-notes summary card with a per-(currency × billing entity) breakdown table, mirroring the Invoice balances treatment that's landing alongside it. Today the customer.creditNotesBalances GraphQL field only groups by currency, so customers transacting across multiple billing entities cannot see their balances split by entity, and there's no per-bucket count of credit notes for the row subtitle.

Description

CustomerCreditNotesBalance now exposes two new non-null fields (billingEntityId: ID! and creditsAvailableCount: Int!) in addition to the existing currency and amountCents. The customer.creditNotesBalances resolver groups finalized credit notes by both currency and billing entity (joining invoices) and emits one row per (currency, billingEntity) pair, returning the sum of balance_amount_cents and the count of finalized credit notes with credit_amount_cents > 0.

The previous balance_amount_cents > 0 filter is removed so buckets with fully-consumed credits still flow through: this is required so the consumer can render the "credit notes issued & credited" subtitle even when the available balance is zero. The consumer applies visibility client-side via amountCents > 0 OR creditsAvailableCount > 0.

Old vs new response shape for the same customer (one billing entity, single currency, and one cross-entity case):

Customer	Before	After
Single-entity, single-currency	[{ currency: "EUR", amountCents: 1500 }]	[{ currency: "EUR", billingEntityId: "...", amountCents: 1500, creditsAvailableCount: 3 }]
Cross-currency, cross-entity (3 buckets)	1 row per currency only	1 row per (currency, billingEntity) pair
Fully-consumed bucket	omitted	included with amountCents: 0 and creditsAvailableCount > 0

Implementation notes:

• Uses PostgreSQL's COUNT(*) FILTER (WHERE ...) rather than COUNT(CASE WHEN ...). Lago is PG-only; the FILTER form is equivalent and more idiomatic.
• Raw SQL aggregates are wrapped in Arel.sql(...) to satisfy Rails 8's disable_joins-style protection against unsafe pluck inputs.
• joins(:invoice) is safe: credit_notes.invoice_id is NOT NULL with a FK, and invoices.billing_entity_id is NOT NULL at the DB level, so no rows are dropped and the new ID! typing is sound.
• schema.graphql and schema.json are regenerated.

How to try locally

1. Check out the branch and rebuild the API container if needed.
2. Seed or open a customer with credit notes across multiple currencies and billing entities (a quick rails console script that creates a couple of :billing_entity, a couple of :invoice, and several :credit_note records works).
3. Run the GraphQL query against the customer:

   query($id: ID!) {
     customer(id: $id) {
       creditNotesBalances {
         currency
         billingEntityId
         amountCents
         creditsAvailableCount
       }
     }
   }

   Confirm: one row per (currency, billingEntityId) pair, amounts sum correctly per bucket, and counts only include credit notes with credit_amount_cents > 0.
4. Verify a single-entity, single-currency customer still returns exactly one row (no regression).
5. bundle exec rspec spec/graphql/types/customers/object_spec.rb spec/graphql/lago_api_schema_spec.rb to confirm the new behavior specs and the schema-dump cross-check both pass.