Variants, step 1a: pulls and runs carry a nullable variant group, and the pull JSON says its group and siblings

Splits #1571 (parked twice: both tries changed how green pulls fold into asks, and test/asks-expiry.test.ts and test/asks-rank.test.ts went red).

Done looks like:

[ ] A named migration adds a nullable variant_group to pulls and runs.
[ ] The pull JSON in the API carries variantGroup (null for an ordinary pull) and variantSiblings (pull numbers sharing the group, empty for an ordinary pull).
[ ] A test (new file added to GATE in scripts/gate.ts, or an existing pulls API test) sets two pulls to one group and reads both siblings; an ordinary pull has null and [].
[ ] Asks, the home and the feed are untouched: no change under src/web/studio/asks.ts or the feed. Run bun test test/asks*.test.ts and the new test before pushing; they must pass unchanged.

Variants, step 1a: pulls and runs carry a nullable variant group, and the pull JSON says its group and siblings #1643