fix: import UploadFile from starlette for beartype compatibility (#741)
beartype sees fastapi.UploadFile and starlette.UploadFile as different
classes even though FastAPI re-exports starlette's class. When FastAPI
parses the multipart form, it creates starlette.UploadFile instances,
but our type hint said fastapi.UploadFile - causing BeartypeCallHintParamViolation.
Also switched to logfire.exception() for better error visibility.
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