Batch Create Measurements

All-or-nothing batch insert. Used by copy/paste and, with link: true, by the RUD-263 linked draw.

Iterates the items inside one transaction; any per-item validation failure surfaces as 422 and the whole transaction rolls back. Order of items in the response matches the request order so the client can map each result back to its originating temp id.

With link: true the items become one locked link group. Per the repo's existing key-becomes-identity idiom (routes/annotated_pdf_exports.py, routes/pdf_markup_imports.py), an Idempotency-Key header BECOMES the group id; without one the server mints a uuid4. A retry carrying the same key finds the group already populated and returns those rows with 200 instead of minting a second group and duplicating N rows.