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.

duty_teller/db/models.py

@@ -26,6 +26,19 @@ class User(Base):
     duties: Mapped[list["Duty"]] = relationship("Duty", back_populates="user")
 
 
+class CalendarSubscriptionToken(Base):
+    """One active calendar subscription token per user; token_hash is unique."""
+
+    __tablename__ = "calendar_subscription_tokens"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    user_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("users.id"), nullable=False
+    )
+    token_hash: Mapped[str] = mapped_column(Text, nullable=False, unique=True)
+    created_at: Mapped[str] = mapped_column(Text, nullable=False)
+
+
 class Duty(Base):
     __tablename__ = "duties"