fix "cannot insert into generated column snapshot.id"

[samuelstroschein]

Context

The snapshot id column being forcibly generated leads to:

• frustration because a simple insert doesn't work
• needs to be explained in the docs
• potentially large performance cost

Proposal

Make the sha256 hash the default for snapshot.id but lift the restriction on snapshot ids being the hash of the content.

• We are not using snapshot ids for comparing changes because it would be flaky.
• The main purpose is data de-duplication.
• As such, it's enough to have a default du-duplicated id.

[samuelstroschein]

The performance cost of forcibly generated snapshot ids is severe.

Copying snapshots from one lix to another or even sending it over the network when syncing could entail hundreds of thousands of snapshots. Even though the snapshot.id has already been computed, the receiver must re-compute the snapshot id.

    const lix1 = await openLix()	

    // potentially 100.000's snapshots
    const snapshots1 = await lix.db.selectFrom("snapshot").selectAll().execute();

	const lix2 = await openLix({});

	await lix2.db
		.insertInto("snapshot")
       // need to re-compute the id for all snapshots
		.values(snapshots1.map((c) => ({ content: c.content })))
		.execute();

	await applyChanges({ lix: lix2, changes: changes1 });