Add retry logic to updateRecoveryWindow to handle concurrent ObjectStore status updates

## Problem

When running scheduled backups with retention policies, we observe transient errors:

```
{"level":"error","msg":"Error while updating the recovery window in the ObjectStore status stanza. Skipping.","error":"Operation cannot be fulfilled on objectstores.barmancloud.cnpg.io \"cluster-name-backup\": the object has been modified; please apply your changes to the latest version and try again"}

{"level":"error","msg":"Retention policy enforcement failed","error":"Operation cannot be fulfilled on objectstores.barmancloud.cnpg.io \"cluster-name-backup\": the object has been modified; please apply your changes to the latest version and try again"}
```

## Root Cause Analysis

After investigating the plugin source code, we identified that the `updateRecoveryWindow` function in `internal/cnpgi/instance/recovery_window.go` performs a direct status update without retry logic:

```go
// recovery_window.go:40
func updateRecoveryWindow(...) error {
// ... builds status ...
return c.Status().Update(ctx, objectStore) // No retry on conflict
}
```

This function is called from two places that can run concurrently:
1. **backup.go:169** - After a backup completes successfully
2. **retention.go:66** - During periodic retention policy enforcement (default every 5 minutes)

When both operations happen close together, Kubernetes optimistic concurrency control rejects one update because the `resourceVersion` changed between read and write.

## Evidence

The same file already has a function that correctly handles this scenario:

```go
// recovery_window.go:65 - setLastFailedBackupTime
func setLastFailedBackupTime(...) error {
return retry.RetryOnConflict(retry.DefaultBackoff, func() error {
var objectStore barmancloudv1.ObjectStore
if err := c.Get(ctx, objectStoreKey, &objectStore); err != nil {
return err
}
// ... update status ...
return c.Status().Update(ctx, &objectStore)
})
}
```

The `setLastFailedBackupTime` function uses `retry.RetryOnConflict` which:
1. Gets a fresh copy of the resource before updating
2. Retries on conflict with exponential backoff

## Impact

- **Severity**: Low - backups complete successfully, status eventually updates
- **User experience**: Confusing error messages in logs
- **Frequency**: Depends on backup/retention timing overlap (we see ~2 errors per 24h)

## Proposed Fix

Apply the same retry pattern to `updateRecoveryWindow`:

```go
func updateRecoveryWindow(
ctx context.Context,
c client.Client,
backupList *catalog.Catalog,
objectStore *barmancloudv1.ObjectStore,
serverName string,
) error {
return retry.RetryOnConflict(retry.DefaultBackoff, func() error {
// Get fresh copy
var freshObjectStore barmancloudv1.ObjectStore
if err := c.Get(ctx, client.ObjectKeyFromObject(objectStore), &freshObjectStore); err != nil {
return err
}

// Build recovery window
convertTime := func(t *time.Time) *metav1.Time {
if t == nil {
return nil
}
return ptr.To(metav1.NewTime(*t))
}

recoveryWindow := freshObjectStore.Status.ServerRecoveryWindow[serverName]
recoveryWindow.FirstRecoverabilityPoint = convertTime(backupList.GetFirstRecoverabilityPoint())
recoveryWindow.LastSuccessfulBackupTime = convertTime(backupList.GetLastSuccessfulBackupTime())

if freshObjectStore.Status.ServerRecoveryWindow == nil {
freshObjectStore.Status.ServerRecoveryWindow = make(map[string]barmancloudv1.RecoveryWindow)
}
freshObjectStore.Status.ServerRecoveryWindow[serverName] = recoveryWindow

return c.Status().Update(ctx, &freshObjectStore)
})
}
```

## Environment

- Plugin version: 0.10.0
- CNPG Operator: 1.26+
- Kubernetes: 1.29+
- Object storage: AWS S3

We're happy to submit a PR if this approach looks correct.

Start in internal/cnpgi/instance/recovery_window.go, read updateRecoveryWindow alongside setLastFailedBackupTime, then inspect its callers in backup.go and retention.go. Confirm the change handles concurrent ObjectStore status updates without the conflict errors described in the issue, while preserving recovery-window updates.

Vom Indexierungsmodell aus dem Issue-Text verfasst.