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

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- refuse an unversioned database that already has a `files` table with
+  a clear schema-version error (2026-09-21, closes
+  https://git.eeqj.de/sneak/sfdupes/issues/11)
+
 - remove the dead `files.dat` references from `Makefile`, `.gitignore`
   and `.dockerignore` (2026-09-21, branch `next`, closes
   https://git.eeqj.de/sneak/sfdupes/issues/22)