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)

README.md

@@ -209,7 +209,10 @@ All three subcommands operate on a single SQLite database file:
     `-shm` files beside it, which SQLite creates with the database
     file's permissions.
 -   Schema (`PRAGMA user_version` is the schema version, currently 1; a
-    database with any other version is a fatal error):
+    database with any other version is a fatal error. A database at
+    version 0 that already has a `files` table was not made by sfdupes;
+    every subcommand refuses it with an error telling the user to remove
+    the file and rescan):
 
     ```sql
     CREATE TABLE files (