refactor: update group duty pin functionality and documentation

- Changed the behavior of the group duty pin feature to send a new message, unpin the old one, and pin the new one instead of editing the existing message. This ensures the pinned message is always fresh.
- Updated the `DUTY_PIN_NOTIFY` configuration description in the documentation to reflect the new message handling approach.
- Revised the architecture documentation to clarify the updated group duty pin process.
- Enhanced tests to verify the new behavior of the group duty pin functionality, ensuring proper message handling and scheduling.

CHANGELOG.md

@@ -11,6 +11,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - **Group duty pin**: when the pinned duty message is updated on schedule, the bot re-pins it so group members get a Telegram notification. Configurable via `DUTY_PIN_NOTIFY` (default: enabled); set to `0` or `false` to only edit the message without re-pinning.
 
+### Changed
+
+- **Group duty pin refresh**: schedule updates now send a new message, unpin the previous one and pin the new one (instead of editing the existing message and re-pinning). Ensures the pinned message is always a fresh post; on pin/unpin errors the stored message_id is not updated so the next run retries.
+
 ## [0.1.0] - 2025-02-20
 
 ### Added