feat: add iCal calendar subscriptions

pyproject.toml

@@ -16,6 +16,8 @@ dependencies = [
   "structlog>=25,<26",
   "lunar-python>=1.2,<2",
   "httpx>=0.28,<1",
+  "icalendar>=6,<7",
+  "python-dateutil>=2.9,<3",
 ]
 
 [dependency-groups]