feat: add calendar subscription token functionality and ICS generation

- Introduced a new database model for calendar subscription tokens, allowing users to generate unique tokens for accessing their personal calendar.
- Implemented API endpoint to return ICS files containing only the subscribing user's duties, enhancing user experience with personalized calendar access.
- Added utility functions for generating ICS files from user duties, ensuring proper formatting and timezone handling.
- Updated command handlers to support the new calendar link feature, providing users with easy access to their personal calendar subscriptions.
- Included unit tests for the new functionality, ensuring reliability and correctness of token generation and ICS file creation.

tests/test_calendar_token_repository.py

@@ -0,0 +1,67 @@
+"""Unit tests for calendar subscription token repository functions."""
+
+import pytest
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker
+
+from duty_teller.db.models import Base
+from duty_teller.db.repository import (
+    create_calendar_token,
+    get_user_by_calendar_token,
+    get_or_create_user_by_full_name,
+)
+
+
+@pytest.fixture
+def session():
+    """In-memory SQLite session with all tables (including calendar_subscription_tokens)."""
+    engine = create_engine(
+        "sqlite:///:memory:", connect_args={"check_same_thread": False}
+    )
+    Base.metadata.create_all(engine)
+    Session = sessionmaker(bind=engine, autocommit=False, autoflush=False)
+    s = Session()
+    try:
+        yield s
+    finally:
+        s.close()
+
+
+@pytest.fixture
+def user_a(session):
+    """Create one user."""
+    u = get_or_create_user_by_full_name(session, "User A")
+    return u
+
+
+def test_create_calendar_token_returns_token_string(session, user_a):
+    """create_calendar_token returns a non-empty URL-safe string."""
+    token = create_calendar_token(session, user_a.id)
+    assert isinstance(token, str)
+    assert len(token) > 0
+    assert " " not in token
+
+
+def test_get_user_by_calendar_token_valid_returns_user(session, user_a):
+    """Valid token returns the correct user."""
+    token = create_calendar_token(session, user_a.id)
+    user = get_user_by_calendar_token(session, token)
+    assert user is not None
+    assert user.id == user_a.id
+    assert user.full_name == "User A"
+
+
+def test_get_user_by_calendar_token_invalid_returns_none(session, user_a):
+    """Invalid or unknown token returns None."""
+    create_calendar_token(session, user_a.id)
+    assert get_user_by_calendar_token(session, "wrong-token") is None
+    assert get_user_by_calendar_token(session, "") is None
+
+
+def test_create_calendar_token_one_active_per_user(session, user_a):
+    """Creating a new token removes the previous one for the same user."""
+    token1 = create_calendar_token(session, user_a.id)
+    token2 = create_calendar_token(session, user_a.id)
+    assert token1 != token2
+    assert get_user_by_calendar_token(session, token1) is None
+    assert get_user_by_calendar_token(session, token2).id == user_a.id