Log the number of imported listens

At first I wanted to have this temporarily to verify whether the
incremental import works, but it's actually nice to have, and the output
of my "count" subcommand is already polluted with other logs, so I might
as well make the importer log this one line.

src/playcount.rs

@@ -416,12 +416,15 @@ impl PlayCounter {
         tx: &mut Transaction,
     ) -> database::Result<()> {
         let start_second = self.last_counted_at.to_posix_timestamp();
+        let mut n = 0;
         for listen_opt in database::iter_listens_since(tx, start_second)? {
             let listen = listen_opt?;
             let at = Instant::from_posix_timestamp(listen.started_at_second);
             let track_id = TrackId(listen.track_id as u64);
             self.count(index, at.into(), track_id);
+            n += 1;
         }
+        println!("Imported {n} new listens from database.");
         Ok(())
     }