Creates an alert for a specific entity (URL or phone number) within Doppel's Brand Protection product.
The organization is determined from the org_id claim in the JWT.

Behavior:

• If an alert already exists for the entity, the existing alert is returned (HTTP 200).
• If the alert exists but with a different brand than requested, returns HTTP 409.
• After creation, the alert is automatically processed through Doppel's triage workflow.
• If source is provided, it must exactly match the "API value" field of an external source configured in your organization's settings in Doppel Vision.