⚡ Bolt: Cache Printify API variant lookups

[merg357]

💡 What: Implemented @functools.lru_cache on the get_variants function across printify_manager.py, create_merch.py, and merch_factory.py to memoize HTTP request results, storing them as immutable tuples.
🎯 Why: In bulk creation loops, querying the same blueprint's variants repeatedly causes an N+1 API call bottleneck. Caching these responses drastically reduces redundant network requests. Returning immutable tuples ensures thread safety and protects the cached data from unintended mutations downstream.
📊 Impact: Reduces redundant N+1 Printify API queries down to a single network call per unique blueprint and provider combination, significantly accelerating product creation times and minimizing API rate limiting.
🔬 Measurement: Verify by executing creation scripts (e.g., python3 create_merch.py) and observing the dramatically reduced network latency when looking up variants for multiple products of the same blueprint.

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PR created automatically by Jules for task 15255439505164766462 started by @merg357

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