Keep the record count and total size in a totals table (closes #146)

info counted the records and added up their sizes with a query that
reads every record, which takes minutes on a large database. The
schema now has a totals table of one row, kept current by three
triggers on files, in the transaction of each insert, delete and size
update. info, scan's load phase and the read phases of report and
trees read that row instead of counting over files. scan writes files
as before.

The schema version stays 1: a database made before this change has no
totals table, and every subcommand fails on it with "no such table:
totals". Remove it and rescan.

Model: opus-5-5

TODO.md

@@ -29,6 +29,13 @@
 
 # Completed Steps
 
+- the database keeps the number of records and the sum of their sizes in the one
+  row of a `totals` table, which three triggers on `files` keep current. `info`
+  reads that row, and so do `scan`'s `load` phase and the `read` phases of
+  `report` and `trees` for their totals; no count reads every record any more. A
+  database made before this has no `totals` table and must be removed and
+  rescanned (2026-10-27, https://git.eeqj.de/sneak/sfdupes/issues/146)
+
 - an operand at or inside a directory named `.zfs` is never scanned, whether
   given alone or under another operand. `scan` warns about it, counts it as
   skipped and does not walk it (2026-10-26,