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.

webapp/style.css

@@ -164,7 +164,9 @@ body {
   transform: none;
 }
 
-.duty-marker {
+.duty-marker,
+.unavailable-marker,
+.vacation-marker {
   display: inline-flex;
   align-items: center;
   justify-content: center;
@@ -173,12 +175,25 @@ body {
   margin-top: 2px;
   font-size: 0.65rem;
   font-weight: 700;
-  color: var(--duty);
-  background: rgba(158, 206, 106, 0.25);
   border-radius: 50%;
   flex-shrink: 0;
 }
 
+.duty-marker {
+  color: var(--duty);
+  background: rgba(158, 206, 106, 0.25);
+}
+
+.unavailable-marker {
+  color: #e0af68;
+  background: rgba(224, 175, 104, 0.25);
+}
+
+.vacation-marker {
+  color: #7dcfff;
+  background: rgba(125, 207, 255, 0.25);
+}
+
 .duty-list {
   font-size: 0.9rem;
 }
@@ -197,6 +212,20 @@ body {
   border-left: 3px solid var(--duty);
 }
 
+.duty-item--unavailable {
+  border-left-color: #e0af68;
+}
+
+.duty-item--vacation {
+  border-left-color: #7dcfff;
+}
+
+.duty-item .duty-item-type {
+  font-size: 0.75rem;
+  color: var(--muted);
+  margin-right: 4px;
+}
+
 .duty-item .name {
   font-weight: 600;
 }