[Product][API] Add last_modified Timestamp to Product API Endpoint

Required fields:

• What is the problem, wish or change?
  The product API endpoint does not return a last_modified date in its response. According to the customer:
  ”Just when the product was last changed, we've built an integration with our CRM and it would be good to check the last changed date of a product when comparing it to the last sync so we're not updating every product every sync, only the ones that changed and we're missed by our webhooks”

• Who or what is impacted?
  Customers using the Salesbuildr API to build integrations or automations that rely on the product endpoint.

• Why does this matter now?
  Customers building integrations with the API have no way to detect product changes without a last_modified field.

• What does “done” look like?
  - The product API endpoint returns a last_modified (or updated_at) timestamp in its response payload.
  - The field is accurately populated and reflects the most recent update to the product record.
  - The field is documented in the API reference.
  - Customers can use the field to filter or sort products by modification date.

Comment by Bart:

• Exclude distributor updates, and include that fact in documentation

Acceptance criteria:

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