Make IFeatureService async end-to-end to eliminate blocking thread-pool calls

[Copilot]

IFeatureService.IsFeatureEnabled was blocking on IFeatureManagerSnapshot.IsEnabledAsync via .GetAwaiter().GetResult(), risking thread-pool starvation and deadlocks under ASP.NET request contexts.

Changes

• IFeatureService / FeatureService — Replaced bool IsFeatureEnabled with Task<bool> IsFeatureEnabledAsync; implementation now returns the Task<bool> directly from IFeatureManagerSnapshot.IsEnabledAsync without blocking
• UploadViewComponent / TabbedNavigationViewComponent — Updated all call sites to await IsFeatureEnabledAsync (both already in async contexts)
• Page models (Index, Completed, Denied, Justification, Review) — Converted OnGet() → async Task OnGetAsync() and awaited feature checks

// Before
public bool IsFeatureEnabled(string featureName)
    => _featureManager.IsEnabledAsync(featureName).GetAwaiter().GetResult();

// After
public Task<bool> IsFeatureEnabledAsync(string featureName)
    => _featureManager.IsEnabledAsync(featureName);

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