⚡ Bolt: Memoize Printify variant API queries

[merg357]

• 💡 What: Added @functools.lru_cache(maxsize=None) to get_variants functions in printify_manager.py, create_merch.py, and merch_factory.py, returning immutable tuples instead of lists.
• 🎯 Why: Fetching blueprint variants repeatedly causes N+1 network bottlenecks during batch product creation loops, even though variants rarely change.
• 📊 Impact: Reduces redundant network API calls significantly, speeding up the bulk creation pipeline.
• 🔬 Measurement: Observe faster execution times when running the pipeline with multiple images for the same blueprint.

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PR created automatically by Jules for task 17692219438116764495 started by @merg357

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