Add --pinned-version / --unpin to datasets update

[zfarrell]

The runtimedb API (`PUT /v1/datasets/{id}`) accepts `pinned_version` (with explicit `null` to unpin), but the CLI's `hotdata datasets update` only exposes `--label` and `--table-name`. Scoped out of #65 to keep that PR focused on the rename use case.

Proposal

Two mutually-exclusive flags:

• `--pinned-version ` — pin to version N
• `--unpin` — remove existing pin (sends JSON `null`)

Wire them through `datasets::update` into the request body. The runtimedb side already has the validation (rejects pin changes that would invalidate existing indexes — see `PinChangeRejectedByIndexes` in `runtimedb/src/engine/datasets.rs`).

Notes

• The response struct `UpdateResponse` (added in feat(datasets): add update subcommand to rename label or table_name #65) already exposes `latest_version` and `pinned_version` for printing.
• Consider also surfacing both in the table-format output so users see the pin state after the update.