Refuse an unversioned database that already has a files table (closes #11)

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)

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- `scan` creates the schema in one transaction; a version-0 database with a
+  `files` table is refused with a clear schema-version error (2026-10-04,
+  https://git.eeqj.de/sneak/sfdupes/issues/11)
+
 - README documents install, Docker, a daily cron scan and how to read
   and check the reports (2026-10-04,
   https://git.eeqj.de/sneak/sfdupes/issues/54)