Update changelog and added a note for comparing dates by string

Author: Taaku18

CHANGELOG.md

@@ -23,7 +23,8 @@ however, insignificant breaking changes do not guarantee a major version bump, s
 - `REGISTRY_PLUGINS_ONLY`, environment variable, when set, restricts to only allow adding registry plugins. ([PR #3247](https://github.com/modmail-dev/modmail/pull/3247))
 - `DISCORD_LOG_LEVEL` environment variable to set the log level of discord.py. ([PR #3216](https://github.com/modmail-dev/Modmail/pull/3216))
 - New registry plugin: [`autoreact`](https://github.com/martinbndr/kyb3r-modmail-plugins/tree/master/autoreact).
-- `STREAM_LOG_FORMAT` and `FILE_LOG_FORMAT` for settings the log format of the stream and file handlers respectively. Possible options are `json` and `plain` (default). ([PR #3305](https://github.com/modmail-dev/Modmail/pull/3305))
+- `STREAM_LOG_FORMAT` and `FILE_LOG_FORMAT` environment variable to set the log format of the stream and file handlers respectively. Possible options are `json` and `plain` (default). ([PR #3305](https://github.com/modmail-dev/Modmail/pull/3305))
+- `LOG_EXPIRATION` environment variable to set the expiration time of logs. ([PR #3257](https://github.com/modmail-dev/Modmail/pull/3257))
 
 ### Changed
 - Repo moved to https://github.com/modmail-dev/modmail.

bot.py

@@ -1713,6 +1713,8 @@ async def log_expiry(self):
 
         now = discord.utils.utcnow()
         expiration_datetime = now - log_expire_after
+        # WARNING: comparison is done lexicographically, not by date.
+        # This is fine as long as the date is in zero-padded ISO format, which it should be.
         expired_logs = await self.db.logs.delete_many({"closed_at": {"$lte": str(expiration_datetime)}})
 
         logger.info(f"Deleted {expired_logs.deleted_count} expired logs.")