Drop `position` from the `asset_usages` unique constraint so the same
asset can't be attached to one target/slot more than once, and update
the insert's onConflict target + lookup to match the narrower key.
Guard it at the service layer too: attachAssetService now rejects an
attach when the asset already has a usage for that target with a new
ASSET_ALREADY_ATTACHED (409) error, backed by an
assetUsageExistsForTarget query.
Rotimi Best
f18e5600f6