Implement external calendar integration and enhance API functionality

- Added support for fetching and parsing external ICS calendars, allowing events to be displayed on the duty grid.
- Introduced a new API endpoint `/api/calendar-events` to retrieve calendar events within a specified date range.
- Updated configuration to include `EXTERNAL_CALENDAR_ICS_URL` for specifying the ICS calendar URL.
- Enhanced the web application to visually indicate days with events and provide event summaries on hover.
- Improved documentation in the README to include details about the new calendar integration and configuration options.
- Updated tests to cover the new calendar functionality and ensure proper integration.

webapp/style.css

@@ -105,6 +105,57 @@ body {
   font-weight: 700;
 }
 
+.day.holiday {
+  background: linear-gradient(135deg, var(--surface) 0%, rgba(187, 154, 247, 0.15) 100%);
+  border: 1px solid rgba(187, 154, 247, 0.35);
+}
+
+.day {
+  position: relative;
+}
+
+.info-btn {
+  position: absolute;
+  top: 0;
+  right: 0;
+  width: 22px;
+  height: 22px;
+  padding: 0;
+  border: none;
+  background: var(--accent);
+  color: var(--bg);
+  font-size: 0.7rem;
+  font-weight: 700;
+  line-height: 1;
+  cursor: pointer;
+  display: inline-flex;
+  align-items: flex-start;
+  justify-content: flex-end;
+  flex-shrink: 0;
+  clip-path: path("M 0 0 L 14 0 Q 22 0 22 8 L 22 22 Z");
+  padding: 2px 3px 0 0;
+}
+
+.info-btn:active {
+  opacity: 0.9;
+}
+
+.calendar-event-hint {
+  position: fixed;
+  z-index: 1000;
+  max-width: 280px;
+  padding: 8px 12px;
+  background: var(--surface);
+  color: var(--text);
+  border-radius: 8px;
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.4);
+  font-size: 0.85rem;
+  line-height: 1.4;
+  white-space: pre-wrap;
+  word-break: break-word;
+  transform: translateY(-100%);
+}
+
 .day-duties {
   font-size: 0.6rem;
   color: var(--duty);