feat(drift): cap the scanner interval at the documented max

intervalMs() enforced the 15-minute floor but not the 1440-minute cap the
settings route and UI apply, so a corrupted or bypass-written DB value
could silently defer an enabled scan indefinitely. Clamp to the same
bounds (fall back to the default outside them). Adds tests for the
over-cap fallback, the isScanning overlap guard, and the route rejecting
an over-cap interval.

backend/src/__tests__/drift-scan-service.test.ts

@@ -88,4 +88,33 @@ describe('DriftScanService', () => {
     await svc.tick(); // within the 60-minute interval => skipped
     expect(spy).toHaveBeenCalledTimes(localNodeCount()); // not doubled
   });
+
+  it('drops a tick that fires while a scan is already running', async () => {
+    db().updateGlobalSetting('drift_scan_enabled', '1');
+    db().updateGlobalSetting('drift_scan_interval_minutes', '60');
+    let release!: () => void;
+    const gate = new Promise<void>(r => { release = r; });
+    const spy = vi.spyOn(DriftLedgerService.getInstance(), 'reconcileNode').mockImplementation(async () => {
+      await gate; // hold the scan open so a second tick overlaps it
+      return { stacks: 0, detected: 0, resolved: 0 };
+    });
+    const svc = DriftScanService.getInstance();
+    const first = svc.tick();      // starts a scan and suspends on the gate (isScanning is now true)
+    await Promise.resolve();
+    await svc.tick();              // fires mid-scan => dropped by the isScanning guard
+    release();
+    await first;
+    expect(spy).toHaveBeenCalledTimes(localNodeCount()); // only the first tick scanned
+  });
+
+  it('falls back to the default interval for an out-of-range or malformed stored value', () => {
+    const svc = DriftScanService.getInstance() as unknown as { intervalMs(): number };
+    const defaultMs = 60 * 60_000;
+    db().updateGlobalSetting('drift_scan_interval_minutes', '99999'); // above the 1440 cap
+    expect(svc.intervalMs()).toBe(defaultMs);
+    db().updateGlobalSetting('drift_scan_interval_minutes', '5'); // below the 15 floor
+    expect(svc.intervalMs()).toBe(defaultMs);
+    db().updateGlobalSetting('drift_scan_interval_minutes', 'banana'); // not a number
+    expect(svc.intervalMs()).toBe(defaultMs);
+  });
 });

backend/src/__tests__/settings-routes.test.ts

@@ -286,6 +286,15 @@ describe('drift detection scan settings', () => {
     expect(DatabaseService.getInstance().getGlobalSettings().drift_scan_interval_minutes).not.toBe('5');
   });
 
+  it('rejects an out-of-range interval above the 1440 minute cap (400)', async () => {
+    const res = await request(app)
+      .post('/api/settings')
+      .set('Cookie', adminCookie)
+      .send({ key: 'drift_scan_interval_minutes', value: '99999' });
+    expect(res.status).toBe(400);
+    expect(DatabaseService.getInstance().getGlobalSettings().drift_scan_interval_minutes).not.toBe('99999');
+  });
+
   it('rejects a non-admin write (403)', async () => {
     const res = await request(app)
       .post('/api/settings')

backend/src/services/DriftScanService.ts

@@ -24,6 +24,7 @@ const BASE_TICK_MS = 60_000;          // re-read settings and check whether a sc
 const INITIAL_DELAY_MS = 45_000;      // let Docker and the node registry settle before the first scan
 const DEFAULT_INTERVAL_MINUTES = 60;
 const MIN_INTERVAL_MINUTES = 15;
+const MAX_INTERVAL_MINUTES = 1440; // matches the settings-route Zod cap; an over-cap or corrupted DB value falls back to the default rather than deferring scans indefinitely
 
 export class DriftScanService {
     private static instance: DriftScanService;
@@ -70,7 +71,7 @@ export class DriftScanService {
     private intervalMs(): number {
         try {
             const raw = Number(DatabaseService.getInstance().getGlobalSettings()['drift_scan_interval_minutes']);
-            const minutes = Number.isFinite(raw) && raw >= MIN_INTERVAL_MINUTES ? raw : DEFAULT_INTERVAL_MINUTES;
+            const minutes = Number.isFinite(raw) && raw >= MIN_INTERVAL_MINUTES && raw <= MAX_INTERVAL_MINUTES ? raw : DEFAULT_INTERVAL_MINUTES;
             return minutes * 60_000;
         } catch {
             return DEFAULT_INTERVAL_MINUTES * 60_000;