fix(typescript): tighten TypeScript null safety and error handling across backend and ops UI

ts/radius/classes.vlan.manager.ts

@@ -104,7 +104,7 @@ export class VlanManager {
     if (this.normalizedMacCache.size > 10000) {
       const iterator = this.normalizedMacCache.keys();
       for (let i = 0; i < 1000; i++) {
-        this.normalizedMacCache.delete(iterator.next().value);
+        this.normalizedMacCache.delete(iterator.next().value!);
       }
     }
 
@@ -348,8 +348,8 @@ export class VlanManager {
         }
         logger.log('info', `Loaded ${data.length} VLAN mappings from storage`);
       }
-    } catch (error) {
-      logger.log('warn', `Failed to load VLAN mappings from storage: ${error.message}`);
+    } catch (error: unknown) {
+      logger.log('warn', `Failed to load VLAN mappings from storage: ${(error as Error).message}`);
     }
   }
 
@@ -364,8 +364,8 @@ export class VlanManager {
     try {
       const mappings = Array.from(this.mappings.values());
       await this.storageManager.setJSON(this.config.storagePrefix, mappings);
-    } catch (error) {
-      logger.log('error', `Failed to save VLAN mappings to storage: ${error.message}`);
+    } catch (error: unknown) {
+      logger.log('error', `Failed to save VLAN mappings to storage: ${(error as Error).message}`);
     }
   }
 }