fix(api): Add fallback image loading from triggers directory

[ManupaKDU]

Added a fallback logic in create_trigger_embedding (in frigate/api/event.py) to search the triggers directory for thumbnail images when an Event object is not found in the database. This logic ensures that valid .webp image triggers are loaded successfully even if the matching Event record has expired or is missing. Also updated error handling to return valid JSONResponse errors when the retrieved event is not an object.

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PR created automatically by Jules for task 841167091730216318 started by @manupawickramasinghe

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