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

TODO.md

@@ -36,12 +36,11 @@
   `head`, `tail`, and `content` all hold the whole-file hash. A file at
   10 MiB or above is gated on the 64 KiB `head` and `tail`, then
   compared on a `content` hash — the whole file below 50 MiB,
-  gigabyte-spaced 1 MiB samples at or above. Schema bumps to version 2
-  (new `content` column); a version 1 database is rejected and must be
-  rescanned, which is required anyway since every stored hash changed.
-  `report` and `trees` group by the extended signature, so the ladder is
-  applied across the whole database. README "Duplicate detection"
-  documents every rung including the probabilistic large-file path.
+  gigabyte-spaced 1 MiB samples at or above. The `content` column is
+  part of the version 1 schema. `report` and `trees` group by the
+  extended signature, so the ladder is applied across the whole
+  database. README "Duplicate detection" documents every rung including
+  the probabilistic large-file path.
 
 - remove the dead `files.dat` references from `Makefile`, `.gitignore`
   and `.dockerignore` (2026-09-21, branch `next`, closes