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.

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. 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.
+- **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 sends a new message, unpins the previous one and pins the new one; if `DUTY_PIN_NOTIFY` is enabled (default), pinning the new message triggers a Telegram notification for members. The first pin (bot added to group or `/pin_duty`) is always silent.
 
 ## Data flow
 

docs/configuration.md

@@ -17,7 +17,7 @@ All configuration is read from the environment (e.g. `.env` via python-dotenv).
 | **CORS_ORIGINS** | comma-separated list | `*` | Allowed origins for CORS. Leave unset or set to `*` for allow-all. Example: `https://your-domain.com`. |
 | **EXTERNAL_CALENDAR_ICS_URL** | string (URL) | *(empty)* | URL of a public ICS calendar (e.g. holidays). If set, those days are highlighted on the duty grid; users can tap "i" on a cell to see the event summary. Empty = no external calendar. |
 | **DUTY_DISPLAY_TZ** | string (timezone name) | `Europe/Moscow` | Timezone for the pinned duty message in groups. Example: `Europe/Moscow`, `UTC`. |
-| **DUTY_PIN_NOTIFY** | `0`, `false`, or `no` to disable | `1` (enabled) | When the pinned duty message is updated on schedule, the bot re-pins it so that group members get a Telegram notification (“Bot pinned a message”). Set to `0`, `false`, or `no` to only edit the message without re-pinning (no notification). The first pin (e.g. when the bot is added to the group or on `/pin_duty`) is always silent. |
+| **DUTY_PIN_NOTIFY** | `0`, `false`, or `no` to disable | `1` (enabled) | When the pinned duty message is updated on schedule, the bot sends a new message, unpins the old one and pins the new one. If enabled, pinning the new message sends a Telegram notification (“Bot pinned a message”). Set to `0`, `false`, or `no` to pin without notification. The first pin (e.g. when the bot is added to the group or on `/pin_duty`) is always silent. |
 | **DEFAULT_LANGUAGE** | `en` or `ru` (normalized) | `en` | Default UI language when the user's Telegram language is unknown. Values starting with `ru` are normalized to `ru`, otherwise `en`. |
 
 ## Roles and access