Generate file UUIDs upfront in checkFileInMemory() rather than
deferring to Files.Create(). This ensures file_chunks and chunk_files
records have valid FileID values when constructed during file
processing, before the batch insert transaction.
Root cause: For new files, file.ID was empty when building the
fileChunks and chunkFiles slices. The ID was only generated later
in Files.Create(), but by then the slices already had empty FileID
values, causing FK constraint failures.
Also adds PROCESS.md documenting the snapshot creation lifecycle,
database transactions, and FK dependency ordering.
