Add total history count to the index API (#1102)

Thought it would be fun to collect some cool stats, maybe put them on
atuin.sh.

atuin-server-postgres/src/lib.rs

@@ -100,6 +100,21 @@ impl Database for Postgres {
         Ok(res.0)
     }
 
+    #[instrument(skip_all)]
+    async fn total_history(&self) -> DbResult<i64> {
+        // The cache is new, and the user might not yet have a cache value.
+        // They will have one as soon as they post up some new history, but handle that
+        // edge case.
+
+        let res: (i64,) = sqlx::query_as("select sum(total) from total_history_count_user")
+            .fetch_optional(&self.pool)
+            .await
+            .map_err(fix_error)?
+            .unwrap_or((0,));
+
+        Ok(res.0)
+    }
+
     #[instrument(skip_all)]
     async fn count_history_cached(&self, user: &User) -> DbResult<i64> {
         let res: (i32,) = sqlx::query_as(