Handle invalid float mtimes in DB check

Likely an add-on or third-party tool created them, and they were breaking
DB queries for a user.

https://sqlite.org/stricttables.html could help us avoid this class of
issue in the future, though we'd need to check what client versions we'd
break with this change, and would need to change the 'sfld is an integer'
hack.

rslib/src/storage/card/fix_mod.sql

@@ -0,0 +1,3 @@
+UPDATE cards
+SET mod = cast(mod AS integer)
+WHERE mod != cast(mod AS integer)

rslib/src/storage/card/mod.rs

@@ -353,6 +353,10 @@ impl super::SqliteStorage {
             .db
             .prepare(include_str!("fix_ivl.sql"))?
             .execute(params![mtime, usn])?;
+        other_cnt += self
+            .db
+            .prepare(include_str!("fix_mod.sql"))?
+            .execute(params![])?;
         other_cnt += self
             .db
             .prepare(include_str!("fix_ordinal.sql"))?