Preflight attachment data to ensure we have enough disk space to write the export

[ReagentX]

Currently, we get the total attachment bytes during the diagnostic phase:

imessage-exporter/imessage-database/src/tables/attachment.rs

Lines 328 to 332 in d2d1eb2

	let mut bytes_query = db
	.prepare(&format!("SELECT SUM(total_bytes) FROM {ATTACHMENT}"))
	.map_err(TableError::Messages)?;
	let total_bytes: i64 = bytes_query.query_row([], |r| r.get(0)).unwrap_or(0);

We can use this data to ensure that the disk has enough free space for the export to run.