perf: Add timestamp to msgs_index7 and speed up Chatlist::try_load() (#7848)

This speeds up chatlisting by 3--4 times on my biggest profile, so the difference is visually
noticeable, particularly after dropping disk caches. Before, `msgs_index2` was used which has less
ordering and requires a full scan of `msgs` table.

src/sql/migrations.rs

@@ -2477,6 +2477,16 @@ UPDATE msgs SET state=24 WHERE state=18; -- Change OutPreparing to OutFailed.
         .await?;
     }
 
+    inc_and_check(&mut migration_version, 156)?;
+    if dbversion < migration_version {
+        sql.execute_migration(
+            "DROP INDEX IF EXISTS msgs_index7;
+            CREATE INDEX msgs_index7 ON msgs (state, hidden, chat_id, timestamp);",
+            migration_version,
+        )
+        .await?;
+    }
+
     let new_version = sql
         .get_raw_config_int(VERSION_CFG)
         .await?