⚡ Bolt: Cache repetitive Printify variants API queries

[merg357]

• 💡 What: Added @functools.lru_cache(maxsize=None) to get_variants in printify_manager.py, merch_factory.py, and create_merch.py and changed their returns to tuples for thread safety.
• 🎯 Why: To solve the N+1 network request problem where the same blueprint variants were requested redundantly for each product design iteration.
• 📊 Impact: Significantly reduces Printify API calls per run, saving network overhead and speeding up batch product creation.
• 🔬 Measurement: Observe faster script execution times and reduced logging of variant API requests during a bulk run.

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PR created automatically by Jules for task 11159538319794880696 started by @merg357

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