⚡ [performance] optimize GCS file metadata fetching

[max-ostapenko]

💡 What:
Removed the file.exists() call in src/controllers/cdnController.js before calling file.getMetadata(). Wrapped file.getMetadata() in a try-catch to properly handle 404 errors when a file doesn't exist.

🎯 Why:
The previous code made two separate network requests to Google Cloud Storage (one to check existence, another to get the metadata). Since getMetadata() inherently checks for existence (throwing a 404 if missing), removing exists() reduces network overhead by exactly half for existing files without changing the functionality.

📊 Measured Improvement:
Created a mock benchmark (src/benchmarks/gcs_metadata.benchmark.js) simulating a 50ms GCS network delay.

• Baseline (exists + getMetadata): ~10100ms for 100 requests.
• Optimized (try-catch getMetadata): ~5030ms for 100 requests.
• Improvement: 50.14% faster for requests where the file exists. For requests where the file does not exist, the performance is identical as both scenarios make exactly 1 network request.

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PR created automatically by Jules for task 18322883415643102784 started by @max-ostapenko

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