Assigns many Shopify resources to one existing template page: the shared-template model (one page, many assignments). Each resource becomes its own assignment row pointing at pageId, so the template content stays shared, not copied. To create an independent, editable copy per resource, use POST /projects/{projectId}/pages instead.

• The target must be a live page assigned to this project. Its type is derived from its existing assignments and must be resource-backed (PRODUCT, COLLECTION, PAGE, BLOG, ARTICLE); any other type returns 400.
• Send 1-100 entries in assignments. handle is the Shopify resource handle (trimmed, kept verbatim); locale defaults to the global assignment ("").
• A resource already assigned to this page is an idempotent no-op, reported under existing.
• All-or-nothing. Every handle is checked first. If any handle already belongs to a different live page, the whole request returns 409 CONFLICT with the clashing handles in details.conflicts, and nothing is written. A resource is never repointed away from another page: unassign it from its current template, then retry.
• New rows are created in a single transaction. All lookups and writes are scoped to projectId.

Required scope: content:write