⚡ Limit concurrent background image downloads in ContentRepository

[Aatricks]

💡 What:

• Added a Semaphore(5) named DOWNLOAD_SEMAPHORE to ContentRepository.
• Modified backgroundCacheImages to wrap downloadAndCacheImage calls with DOWNLOAD_SEMAPHORE.withPermit { ... }.
• Added a new test ContentRepositoryConcurrencyTest.kt to reproduce the high concurrency issue and verify the fix.

🎯 Why:

• Previously, backgroundCacheImages launched a coroutine for every image in a chapter immediately. For chapters with many images (e.g., Manhwa/Manga), this could trigger hundreds of concurrent network requests, potentially overwhelming the network connection, causing timeouts, or consuming excessive system resources.
• Limiting concurrency to 5 ensures steady background downloading without blocking the UI or other critical network tasks.

📊 Measured Improvement:

• Baseline: A reproduction test with 50 images showed a peak of 50 concurrent requests (unbounded).
• Optimized: The same test with the fix showed a peak of 5 concurrent requests (strictly limited by the semaphore).
• This confirms the optimization effectively throttles the background workload.

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PR created automatically by Jules for task 12468815160325834198 started by @Aatricks

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