⚡ Optimize LazyThumbnail stats with caching

[Snider]

💡 What:

• Implemented a cache-aside strategy for LazyThumbnail::getStats to avoid iterating files on every request.
• Added logic to incrementally update stats cache on generate, delete, and purgeStale.
• Added a "dirty" flag mechanism to handle race conditions where cache is missing during generation.
• Added a benchmark test to verify performance and correctness.

🎯 Why:

• getStats was performing O(N) file operations (listing and sizing all files), which is extremely slow on large datasets or remote storage (S3).
• This optimization reduces the complexity to O(1) for subsequent requests.

📊 Measured Improvement:

• Baseline (1000 files in memory): ~44-65ms
• Optimized: ~0.18ms
• Improvement: >200x speedup (even more significant on real storage).

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PR created automatically by Jules for task 9053527642049529520 started by @Snider

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[github-actions]

QA Pipeline Results

Check	Status	Details
Tests	✓	209 tests, 419 assertions
PHPStan	✓	0 errors
Psalm	✗	1 issues
Code Style	✓	0 files need formatting
Security	✓	0 vulnerabilities

Artifacts

• test-results.xml - JUnit test results
• phpstan.json - PHPStan analysis
• psalm.json / psalm.sarif - Psalm analysis
• pint.json - Code style report
• audit.json - Security audit

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Generated by core php qa pipeline