Add OCR feature flag and guard flows when disabled

[ohld]

Summary

• add an OCR_ENABLED configuration flag and expose it through settings
• skip OCR steps in meme storage flows when the feature is disabled while keeping the final pipeline running
• relax storage service filters and Prefect deployment defaults so OCR-less memes can publish and operators understand how to re-enable OCR

Testing

• pytest (fails: missing optional dependencies such as pytest_asyncio, sqlalchemy, and orjson)

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https://chatgpt.com/codex/tasks/task_e_68e25811276c83268234202e0e3c701f