Custom Age Threshold On Cache Objects Over Capacity

## 🏗 Enhancement Proposal

Allow configuring the age threshold used when cleaning up cache objects over capacity.

### Pitch

Currently, `getObjectsOverCapacity` uses a hardcoded `Duration(days: 1)` to filter which cached objects are eligible for removal during cleanup. This means objects touched within the last 24 hours are never removed, even if the cache exceeds its capacity.

This can be problematic in scenarios where:
- Apps have high cache turnover and need more aggressive cleanup (e.g., `Duration(hours: 6)`)
- Apps want to preserve recently accessed items for longer periods (e.g., `Duration(days: 7)`)
- The 1-day threshold doesn't align with the app's usage patterns

**Proposed solution:**

Add an optional `maxCapacityAgeFilterDuration` parameter to `Config` that allows developers to customize this threshold. The parameter defaults to `Duration(days: 1)` to maintain backward compatibility.

```dart
CacheManager(
Config(
'customCacheKey',
maxCapacityAgeFilterDuration: Duration(hours: 12), // Only remove items untouched for 12+ hours
),
);
```

This gives developers fine-grained control over cache eviction behavior without changing the default experience.

### Platforms affected (mark all that apply)
- [x] :iphone: iOS
- [x] :robot: Android

Locate Config and getObjectsOverCapacity, then trace where the hardcoded one-day Duration is used during cleanup. Add the optional duration while preserving the one-day default, support custom values such as 12 hours, and verify cache cleanup still respects the configured threshold.

Written by the indexing model from the issue text.