Add event type handling for duties in the system

- Introduced a new `event_type` column in the `duties` table to categorize duties as 'duty', 'unavailable', or 'vacation'.
- Updated the duty schedule import functionality to parse and store event types from the JSON input.
- Enhanced the API response to include event types for each duty, improving the calendar display logic.
- Modified the web application to visually differentiate between duty types in the calendar and duty list.
- Updated tests to cover new event type functionality and ensure correct parsing and storage of duties.
- Revised README documentation to reflect changes in duty event types and their representation in the system.

alembic/versions/003_duties_event_type.py

@@ -0,0 +1,28 @@
+"""Duties: add event_type (duty | unavailable | vacation)
+
+Revision ID: 003
+Revises: 002
+Create Date: 2025-02-17
+
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "003"
+down_revision: Union[str, None] = "002"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "duties",
+        sa.Column("event_type", sa.Text(), nullable=False, server_default="duty"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("duties", "event_type")