Advanced RxJS for HTTP: retry, backoff, and caching

Production APIs sometimes fail temporarily. Your app should handle it gracefully.

## 1) retry with delay (simple backoff)

```ts
import { retry, delay, of, throwError } from 'rxjs';

this.http.get('/api/data').pipe(
  retry({
    count: 2,
    delay: (_err, retryCount) => of(null).pipe(delay(500 * retryCount)),
  })
);
```

This retries up to 2 times with increasing delay.

## 2) Basic caching pattern

If multiple components request same data, caching avoids repeated calls.

```ts
import { shareReplay } from 'rxjs';

private products$ = this.http.get('/api/products').pipe(
  shareReplay(1)
);

getProducts() {
  return this.products$;
}
```

## Why shareReplay(1) helps

- first subscriber triggers request
- later subscribers reuse cached response

## Warning

Cache invalidation is real. If data changes often, add refresh logic.

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