Keep the database schema at version 1 (closes #61)

sfdupes is pre-1.0, with no installed base and no databases anywhere,
so the schema is changed in place and its version stays 1.
schemaVersion goes back to 1; the six-column files table, content
included, is the version 1 schema. The check that stops on a database
with any other version stays. README.md and TODO.md no longer describe
a version 2 or rejecting and rescanning version 1 databases. The
main.go package comment still described 1024-byte end windows and said
full file contents are never read; it now describes the hashes the
code computes.

Model: opus-5-5

README.md

@@ -154,11 +154,8 @@ All three subcommands operate on a single SQLite database file:
     that dies partway leaves a valid database holding everything
     hashed so far; the next scan skips those records and converges
     toward the filesystem.
--   Schema (`PRAGMA user_version` is the schema version, currently 2; a
-    database with any other version is a fatal error). Version 2 added
-    the `content` column and the 64 KiB head/tail signature (replacing
-    the version 1 1 KiB end windows), so a version 1 database cannot be
-    reused: it is rejected and the tree must be rescanned from scratch.
+-   Schema (`PRAGMA user_version` is the schema version, currently 1; a
+    database with any other version is a fatal error):
 
     ```sql
     CREATE TABLE files (