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 not
created by sfdupes: a foreign or partially initialized file. 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
check for a files table at version 0 and refuse such a database with
the schema-version error telling the operator to remove the file and
rescan. A genuinely empty database is unchanged: scan creates the
schema, report and trees refuse it 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 (2026-10-04,
+  https://git.eeqj.de/sneak/sfdupes/issues/11)
+
 - the `Dockerfile` build stage keeps the Go module cache out of `builder`'s
   home and copies the sources with `--chown`, so no `chown -R` walks them
   (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/43)