Phase 0/2: API layer kept 1:1, only secrets.h include path fixed

http_client/weather_api/calendar_api/departures_api are hardware-
independent (HTTP GET + ArduinoJson parsing, same endpoints, same
JSON). Kept verbatim per PLAN.md migration strategy; only changed
#include "../../include/secrets.h" -> #include <secrets.h> to match
the PlatformIO include/ layout (no functional change).

src/api/calendar_api.h

@@ -0,0 +1,19 @@
+#pragma once
+#include <Arduino.h>
+#include <vector>
+
+struct CalendarEvent {
+    long id = 0;
+    String summary;
+    String startAt;
+    String endAt;
+    bool allDay = false;
+    String status;
+};
+
+struct CalendarData {
+    bool valid = false;
+    std::vector<CalendarEvent> events;
+};
+
+CalendarData fetchCalendar();