⚡ Bolt: Caching get_variants for performance

[merg357]

💡 What: Added @functools.lru_cache to get_variants in printify_manager.py, merch_factory.py, and create_merch.py, and casted the returned lists to immutable tuples.
🎯 Why: To prevent duplicate N+1 network API calls for the exact same blueprint and provider combination during product creation loops.
📊 Impact: Significantly speeds up product creation by eliminating redundant HTTP requests.
🔬 Measurement: Run product creation commands and observe faster processing times due to cached API responses.

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PR created automatically by Jules for task 14106385344589799019 started by @merg357

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