⚡ Bolt: Memoize Printify variants API to fix N+1 bottleneck

[merg357]

💡 What: Added @functools.lru_cache(maxsize=None) to get_variants in printify_manager.py and changed its return type from a mutable list to an immutable tuple.\n🎯 Why: Iterative product creation loops repeatedly fetched the same blueprint variants from the Printify API for each uploaded image, creating an N+1 network bottleneck.\n📊 Impact: Eliminates redundant HTTP requests for identical blueprint/provider combinations, significantly speeding up bulk product creation processes.\n🔬 Measurement: Verify by running python3 printify_manager.py create with multiple designs and observing faster processing times without redundant API calls for variants.

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PR created automatically by Jules for task 10547425520696752642 started by @merg357

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