⚡ Bolt: Cache Printify blueprint variants to prevent N+1 API calls

[merg357]

💡 What: Memoized get_variants using @functools.lru_cache and updated its return type to an immutable tuple.
🎯 Why: Prevented an N+1 API call bottleneck where identical blueprint/provider combinations were fetched repeatedly during bulk product creation in cmd_create.
📊 Impact: Reduces network requests by avoiding redundant Printify API calls, significantly speeding up the bulk creation process.
🔬 Measurement: Run the script using python3 printify_manager.py run. Measure execution time and verify that subsequent product creations using the same blueprint/provider reuse the cached variants.

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PR created automatically by Jules for task 9044183134636734626 started by @merg357

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