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/weather_api.h

@@ -0,0 +1,27 @@
+#pragma once
+#include <Arduino.h>
+#include <vector>
+
+struct WeatherCurrent {
+    int temperature = 0;
+    String symbol;
+    String description;
+};
+
+struct ForecastEntry {
+    String time;
+    int temperature = 0;
+    String symbol;
+    String description;
+    float precipitationProbability = 0.0f;
+    String precipitationType;
+};
+
+struct WeatherData {
+    bool valid = false;
+    WeatherCurrent current;
+    std::vector<ForecastEntry> forecast;
+};
+
+WeatherData fetchWeather();
+bool willRainSoon(const WeatherData &data, int hours = 8);