Core API: find newsletter by email address

🔍 Problem Statement
As a developer, I experience the Core API only allowing newsletter retrieval by ID, which makes it impossible to check or manage newsletter subscriptions for a given email address without first knowing the subscription ID.

💡 User Story
As a developer, I want to query the Core API for newsletter subscriptions by email address, so that I can programmatically check opt-in status and manage newsletters for any customer.

🎯 Definition of Done
- Given a valid email address, when calling GET /v2/newsletters with an email filter, then matching newsletter subscriptions are returned.
- This change impacts the API (newsletter endpoint — add email query param). New endpoint must be documented and tested.
- Limitations: returns subscriptions for the authenticated org only; multiple subscriptions per email are returned as a list.

Currently the Core API only allows newsletters to be retrieved by id. It would be useful to be able to retrieve newsletters by email address. Example use case: A company using Pelcro is starting a email newsletter. The marketing department has a list of email addresses they would like to opt-in to this new newsletter. Using the Core API, I would like to do the following: For each email address: Look up an existing newsletter object in Pelcro with a matching email address. If a newsletter object exists, append a new list id to the lists field and save this to Pelcro (HTTP PUT). If a newsletter object does not exist, create a new object and set lists to the new list id (HTTP POST).