feat: add group duty pin notification feature

- Introduced a new configuration option `DUTY_PIN_NOTIFY` to control whether the bot re-pins the duty message when updated, providing notifications to group members.
- Updated the architecture documentation to reflect the new functionality of re-pinning duty messages.
- Enhanced the `.env.example` file to include the new configuration option with a description.
- Added tests to verify the behavior of the new refresh pin command and its integration with the existing group duty pin functionality.
- Updated internationalization messages to include help text for the new `/refresh_pin` command.

docs/architecture.md

@@ -8,7 +8,7 @@ High-level architecture of Duty Teller: components, data flow, and package relat
 - **FastAPI** — HTTP server: REST API (`/api/duties`, `/api/calendar-events`, `/api/calendar/ical/{token}.ics`) and static miniapp at `/app`. Runs in a separate thread alongside the bot.
 - **Database** — SQLAlchemy ORM with Alembic migrations. Default backend: SQLite (`data/duty_teller.db`). Stores users, duties (with event types: duty, unavailable, vacation), group duty pins, calendar subscription tokens.
 - **Duty-schedule import** — Two-step admin flow: handover time (timezone → UTC), then JSON file. Parser produces per-person date lists; import service deletes existing duties in range and inserts new ones.
-- **Group duty pin** — In groups, the bot can pin the current duty message; time/timezone for the pinned text come from `DUTY_DISPLAY_TZ`. Pin state is restored on startup from the database.
+- **Group duty pin** — In groups, the bot can pin the current duty message; time/timezone for the pinned text come from `DUTY_DISPLAY_TZ`. Pin state is restored on startup from the database. When the duty changes on schedule, the bot edits the pinned message and, if `DUTY_PIN_NOTIFY` is enabled (default), re-pins it so that members get a Telegram notification; the first pin (bot added to group or `/pin_duty`) is always silent.
 
 ## Data flow