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/http_client.h

@@ -0,0 +1,10 @@
+#pragma once
+#include <Arduino.h>
+
+struct ApiResult {
+    bool success = false;
+    String body;
+    int httpCode = -1;
+};
+
+ApiResult httpGet(const String &url, uint32_t timeoutMs = 5000);