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

ts/cache/classes.cached.document.ts

@@ -22,7 +22,7 @@ export abstract class CachedDocument<T extends CachedDocument<T>> extends plugin
    * Timestamp when the document expires and should be cleaned up
    * NOTE: Subclasses must add @svDb() decorator
    */
-  public expiresAt: Date;
+  public expiresAt!: Date;
 
   /**
    * Timestamp of last access (for LRU-style eviction if needed)