⚡ Bolt: Memoize Printify variant API queries

[merg357]

💡 What: Added @functools.lru_cache(maxsize=None) to get_variants in printify_manager.py, create_merch.py, and merch_factory.py to memoize the Printify API responses. Changed the return type to an immutable tuple[int, ...] to guarantee thread-safety.
🎯 Why: Iterative product creation loops repeatedly query the identical blueprint and print provider IDs, causing an N+1 network bottleneck.
📊 Impact: Eliminates redundant API calls during bulk merchandise creation, drastically reducing total execution time.
🔬 Measurement: Verified that multiple creations using the same blueprint will trigger only one network request per unique blueprint/provider combo.

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PR created automatically by Jules for task 15644785792414516019 started by @merg357

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