⚡ Bolt: Cache ML models in memory to prevent reload bottleneck

[benpiper]

💡 What: Added @functools.lru_cache(maxsize=1) to whisper.load_model and load_rvm_model.
🎯 Why: Prevent severe performance bottlenecks and OOM issues from repeatedly loading large models from disk for each API call.
📊 Impact: Eliminates multi-second model loading time on subsequent processing requests.
🔬 Measurement: Check the logs when processing multiple videos; the model loading message should only appear once per model.

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PR created automatically by Jules for task 3443362581049218530 started by @benpiper

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