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.

db/schemas.py

@@ -1,5 +1,7 @@
 """Pydantic schemas for API and validation."""
 
+from typing import Literal
+
 from pydantic import BaseModel, ConfigDict
 
 
@@ -38,9 +40,10 @@ class DutyInDb(DutyBase):
 
 
 class DutyWithUser(DutyInDb):
-    """Duty with full_name for calendar display."""
+    """Duty with full_name and event_type for calendar display."""
 
     full_name: str
+    event_type: Literal["duty", "unavailable", "vacation"] = "duty"
 
     model_config = ConfigDict(from_attributes=True)