Implement group duty pinning and user phone management

- Added functionality to pin duty messages in group chats, including scheduling updates and handling bot add/remove events.
- Introduced a new `GroupDutyPin` model to store pinned message details and a `phone` field in the `User` model for user contact information.
- Implemented commands for users to set or clear their phone numbers in private chats.
- Enhanced the repository with functions to manage group duty pins and user phone data.
- Updated handlers to register new commands and manage duty pin updates effectively.

.env.example

@@ -12,3 +12,6 @@ ADMIN_USERNAMES=admin1,admin2
 
 # Optional: URL of a public ICS calendar (e.g. holidays). Days from this calendar are highlighted on the duty grid; click "i" for summary.
 # EXTERNAL_CALENDAR_ICS_URL=https://example.com/holidays.ics
+
+# Timezone for the pinned duty message in groups (e.g. Europe/Moscow).
+# DUTY_DISPLAY_TZ=Europe/Moscow