fix(migration): detect flat structure in upsDevices for proper v3→v4 migration

The previous migration only checked for upsList field, but saveConfig()
strips upsList when saving, creating a race condition. If the daemon
restarted with a partially-migrated config (upsDevices with flat structure),
the migration wouldn't run because it only looked for upsList.

Now shouldRun() also detects:
- upsDevices with flat structure (host at top level, no snmp object)

And migrate() handles both:
- config.upsList (pure v3)
- config.upsDevices with flat structure (partially migrated)

This fixes the "Cannot read properties of undefined (reading 'host')"
error that occurred when configs had upsDevices but flat structure.

ts/migrations/migration-v3-to-v4.ts

@@ -44,17 +44,30 @@ export class MigrationV3ToV4 extends BaseMigration {
   readonly toVersion = '4.0';
 
   async shouldRun(config: any): Promise<boolean> {
-    // V3 format has upsList instead of upsDevices
-    return !!config.upsList && !config.upsDevices;
+    // V3 format has upsList OR has upsDevices with flat structure (host at top level)
+    if (config.upsList && !config.upsDevices) {
+      return true; // Classic v3 with upsList
+    }
+
+    // Check if upsDevices exists but has flat structure (v3 format)
+    if (config.upsDevices && config.upsDevices.length > 0) {
+      const firstDevice = config.upsDevices[0];
+      // V3 has host at top level, v4 has it nested in snmp object
+      return !!firstDevice.host && !firstDevice.snmp;
+    }
+
+    return false;
   }
 
   async migrate(config: any): Promise<any> {
     logger.info(`${this.getName()}: Migrating v3 config to v4 format...`);
-    logger.dim(`  - Renaming upsList → upsDevices`);
     logger.dim(`  - Restructuring UPS devices (flat → nested snmp config)`);
 
+    // Get devices from either upsList or upsDevices (for partially migrated configs)
+    const sourceDevices = config.upsList || config.upsDevices;
+
     // Transform each UPS device from v3 flat structure to v4 nested structure
-    const transformedDevices = config.upsList.map((device: any) => {
+    const transformedDevices = sourceDevices.map((device: any) => {
       // Build SNMP config object
       const snmpConfig: any = {
         host: device.host,