IdentityBindTier dbBackfill contract cleanup

audit-venus-ca non-blocking finding on fe316ed: IdentityBindTier includes dbBackfill tier but logIdentityBound hardcodes retroactive:false/sameTxn:true for every tier regardless — no current dbBackfill caller exists so it doesn't invalidate the fix, but the contract is inconsistent. Remove the dbBackfill tier until a caller needs it, or make retroactive/sameTxn fields explicit per-tier before first use.