Skip BU history entries when previous_bu is unknown

Only record BU reassignment in ivanti_finding_bu_history when the
previous_bu is a known managed BU (from EXPECTED_BUS). Findings that
were never in our sync cache show as UNKNOWN which provides no
actionable insight for asset movement tracking.

Closes #28

backend/routes/ivantiFindings.js

@@ -798,12 +798,17 @@ async function runBUDriftChecker(newlyArchivedIds, apiKey, clientId, skipTls, pr
         if (classification === 'bu_reassignment' && found) {
             try {
                 // Determine previous BU from the pre-sync snapshot (passed in from syncFindings)
-                const previousBu = (previousBuMap && previousBuMap.get(id)) || 'UNKNOWN';
+                const previousBu = (previousBuMap && previousBuMap.get(id)) || '';
+
+                // Only record if we have a known previous BU — "UNKNOWN → X" entries
+                // provide no actionable insight for asset movement tracking.
+                if (previousBu && EXPECTED_BUS.has(previousBu)) {
                     await pool.query(
                         `INSERT INTO ivanti_finding_bu_history (finding_id, finding_title, host_name, previous_bu, new_bu, detected_at)
                          VALUES ($1, $2, $3, $4, $5, NOW())`,
                         [id, found.title || '', found.hostName || '', previousBu, found.bu]
                     );
+                }
             } catch (err) {
                 console.error(`[BU Drift Checker] Error recording BU change for finding ${id}:`, err.message);
             }