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.cpp

@@ -0,0 +1,25 @@
+#include "http_client.h"
+#include <HTTPClient.h>
+#include <WiFi.h>
+
+ApiResult httpGet(const String &url, uint32_t timeoutMs) {
+    ApiResult result;
+    if (WiFi.status() != WL_CONNECTED) {
+        result.success = false;
+        result.httpCode = -1;
+        return result;
+    }
+    HTTPClient http;
+    http.setTimeout(timeoutMs);
+    http.begin(url);
+    int code = http.GET();
+    result.httpCode = code;
+    if (code == HTTP_CODE_OK) {
+        result.body = http.getString();
+        result.success = true;
+    } else {
+        result.success = false;
+    }
+    http.end();
+    return result;
+}