Implement duty schedule import functionality and enhance user management

- Added a new command `/import_duty_schedule` for importing duty schedules via JSON, restricted to admin users.
- Introduced a two-step import process: specifying handover time and uploading a JSON file.
- Updated the database schema to allow `telegram_user_id` to be nullable for user creation by full name.
- Implemented repository functions for user management, including `get_or_create_user_by_full_name` and `delete_duties_in_range`.
- Enhanced README documentation with details on the new import command and JSON format requirements.
- Added comprehensive tests for the duty schedule parser and integration tests for the import functionality.

importers/__init__.py

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+"""Importers for duty data (e.g. duty-schedule JSON)."""

importers/duty_schedule.py

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+"""Parser for duty-schedule JSON format. No DB access."""
+
+import json
+from dataclasses import dataclass
+from datetime import date, timedelta
+
+# Символы, обозначающие день дежурства в ячейке duty (CSV с разделителем ;)
+DUTY_MARKERS = frozenset({"б", "Б", "в", "Н", "О"})
+
+
+@dataclass
+class DutyScheduleResult:
+    """Parsed duty schedule: start_date, end_date, and per-person duty dates."""
+
+    start_date: date
+    end_date: date
+    entries: list[tuple[str, list[date]]]  # (full_name, list of duty dates)
+
+
+class DutyScheduleParseError(Exception):
+    """Invalid or missing fields in duty-schedule JSON."""
+
+    pass
+
+
+def parse_duty_schedule(raw_bytes: bytes) -> DutyScheduleResult:
+    """Parse duty-schedule JSON. Returns start_date, end_date, and list of (full_name, duty_dates).
+
+    - meta.start_date (YYYY-MM-DD) and schedule (array) required.
+    - meta.weeks optional; number of days from max duty string length (split by ';').
+    - For each schedule item: name (required), duty = CSV with ';'; index i = start_date + i days.
+    - Cell value after strip in DUTY_MARKERS => duty day.
+    """
+    try:
+        data = json.loads(raw_bytes.decode("utf-8"))
+    except (json.JSONDecodeError, UnicodeDecodeError) as e:
+        raise DutyScheduleParseError(f"Invalid JSON or encoding: {e}") from e
+
+    meta = data.get("meta")
+    if not meta or not isinstance(meta, dict):
+        raise DutyScheduleParseError("Missing or invalid 'meta'")
+
+    start_str = meta.get("start_date")
+    if not start_str or not isinstance(start_str, str):
+        raise DutyScheduleParseError("Missing or invalid meta.start_date")
+    try:
+        start_date = date.fromisoformat(start_str.strip())
+    except ValueError as e:
+        raise DutyScheduleParseError(f"Invalid meta.start_date: {start_str}") from e
+
+    schedule = data.get("schedule")
+    if not isinstance(schedule, list):
+        raise DutyScheduleParseError("Missing or invalid 'schedule' (must be array)")
+
+    max_days = 0
+    entries: list[tuple[str, list[date]]] = []
+
+    for row in schedule:
+        if not isinstance(row, dict):
+            raise DutyScheduleParseError("schedule item must be an object")
+        name = row.get("name")
+        if name is None or not isinstance(name, str):
+            raise DutyScheduleParseError("schedule item must have 'name' (string)")
+        full_name = name.strip()
+        if not full_name:
+            raise DutyScheduleParseError("schedule item 'name' cannot be empty")
+
+        duty_str = row.get("duty")
+        if duty_str is None:
+            duty_str = ""
+        if not isinstance(duty_str, str):
+            raise DutyScheduleParseError("schedule item 'duty' must be string")
+
+        cells = [c.strip() for c in duty_str.split(";")]
+        max_days = max(max_days, len(cells))
+
+        duty_dates: list[date] = []
+        for i, cell in enumerate(cells):
+            if cell in DUTY_MARKERS:
+                d = start_date + timedelta(days=i)
+                duty_dates.append(d)
+        entries.append((full_name, duty_dates))
+
+    if max_days == 0:
+        end_date = start_date
+    else:
+        end_date = start_date + timedelta(days=max_days - 1)
+
+    return DutyScheduleResult(start_date=start_date, end_date=end_date, entries=entries)