#11619 fix(subscription): cspell ignores bookkept / Finishable in the billing service and its spec

`Check Spelling and Typos with cspell` is red on develop since #11570 (`a43a16ec95`). It flags the identifiers `bookkeptMetadata`, `BOOKKEPT_*` and `isFinishable` in `subscription.service.ts` and `subscription.service.spec.ts`.

This adds one file-level `// cspell:ignore bookkept Finishable` to each of the two files, keeping the exception local (the same approach the ever-connect specs use). No code change.

Verified: `cspell` on both files, 0 issues.

🤖 Generated with Claude Code

---

Summary by cubic

Fixes the spell-check job on develop by adding file-level cspell:ignore directives for bookkept and Finishable in the billing service and its spec.

• No code changes; only the two files get the local spelling exception, keeping the ignore scope contained.