⚡ Bolt: Memoize static Printify API calls to prevent N+1 network requests

[merg357]

💡 What: Added @functools.lru_cache to get_variants functions across create_merch.py, merch_factory.py, and printify_manager.py to memoize repetitive network calls. Replaced returned lists with immutable tuples to guarantee safety.
🎯 Why: Within iterative product creation loops, querying the identical blueprint variants on each iteration resulted in a severe N+1 bottleneck. Since blueprint variants are static, redundant queries needlessly blocked execution.
📊 Impact: Eliminates N+1 network requests. Measurably speeds up bulk product creation processing.
🔬 Measurement: Re-run a bulk product creation command and observe significantly reduced total script execution time and network I/O.

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PR created automatically by Jules for task 7782794411154088461 started by @merg357

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