A database at user_version 0 that already has a files table was made
by something else: scan used to run its CREATE TABLE on it and fail
with a raw SQLite error, and report and trees gave only a bare version
mismatch. All three now refuse such a database with the schema-version
error telling the operator to remove the file and rescan. scan creates
the table and index and sets the version in one transaction, so a first
scan stopped partway leaves an empty database the next scan sets up,
never a files table at version 0. A genuinely empty database is
unchanged.
Model: opus-4-8 (implementation); opus-5-5 (rebase)
sneak