⚡ Bolt: Memoize Printify variants API to fix N+1 bottleneck

[merg357]

💡 What: Added @functools.lru_cache to get_variants in printify_manager.py and changed the return type to an immutable tuple.
🎯 Why: Iterative product creation caused an N+1 network bottleneck by repeatedly fetching static blueprint variants via API for every single product generated.
📊 Impact: Significantly speeds up product creation (batch processing) by eliminating redundant API calls for the same blueprint and provider.
🔬 Measurement: Run python3 printify_manager.py run with multiple images and measure the time taken to create products; you will notice reduced network delay and faster execution.

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PR created automatically by Jules for task 7715853405841125248 started by @merg357

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