Remove deprecated snapshot system

The register/document model has fully replaced the snapshot system.
Delete every snapshot-scoped row and strip the application code that
referenced them: SnapshotID/SourceID struct fields, snapshot_id IS NULL
filters, snapshot columns from SELECT/INSERT statements and named args,
and the eight migrate-*-snapshots-to-documents one-shot tools.

The remaining snapshot_id / source_id columns, the snapshots and
controls_snapshots tables, the snapshots_type enum, and the
snapshot-scoped indexes are now unused; they are dropped in a follow-up
schema migration so this change can roll back cleanly without leaving
orphaned data.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

contrib/claude/coredata.md

@@ -13,7 +13,6 @@ Every entity uses `gid.GID` for its ID, `db` tags for pgx mapping, and `CreatedA
 type (
 	Asset struct {
 		ID             gid.GID   `db:"id"`
-		SnapshotID     *gid.GID  `db:"snapshot_id"`
 		Name           string    `db:"name"`
 		OrganizationID gid.GID   `db:"organization_id"`
 		AssetType      AssetType `db:"asset_type"`