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

An unversioned database (user_version 0) that already contains a files
table was not created by this build; it is a foreign or partially
initialized file. Adopting it silently could corrupt unrelated data, so
createSchema now checks for a files table first and, when one exists,
returns the schema-version error telling the operator to remove the file
and rescan. A genuinely empty database is still created and stamped as
before.

Model: opus-4-8 (implementation); opus-5-5 (rebase)

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- refuse an unversioned database that already has a `files` table with
+  a clear schema-version error (2026-10-03,
+  https://git.eeqj.de/sneak/sfdupes/issues/11)
+
 - escape tabs, newlines, carriage returns and backslashes in report,
   trees and warning paths; the root directory's path is `/`
   (2026-10-03, https://git.eeqj.de/sneak/sfdupes/issues/7)