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.cpp

@@ -0,0 +1,44 @@
+#include "weather_api.h"
+#include "http_client.h"
+#include <secrets.h>
+#include <ArduinoJson.h>
+
+WeatherData fetchWeather() {
+    WeatherData data;
+    ApiResult res = httpGet(WEATHER_API_URL);
+    if (!res.success) { data.valid = false; return data; }
+
+    DynamicJsonDocument doc(8192);
+    if (deserializeJson(doc, res.body)) { data.valid = false; return data; }
+
+    JsonObject current = doc["current"];
+    data.current.temperature = current["temperature"] | 0;
+    data.current.symbol = current["symbol"] | "";
+    data.current.description = current["description"] | "";
+
+    JsonArray forecast = doc["forecast"];
+    for (JsonObject entry : forecast) {
+        ForecastEntry fe;
+        fe.time = entry["time"] | "";
+        fe.temperature = entry["temperature"] | 0;
+        fe.symbol = entry["symbol"] | "";
+        fe.description = entry["description"] | "";
+        JsonObject precip = entry["precipitation"];
+        fe.precipitationProbability = precip["probability"] | 0.0f;
+        fe.precipitationType = precip["type"] | "";
+        data.forecast.push_back(fe);
+    }
+    data.valid = true;
+    return data;
+}
+
+bool willRainSoon(const WeatherData &data, int hours) {
+    if (!data.valid) return false;
+    int checked = 0;
+    for (const auto &entry : data.forecast) {
+        if (checked >= hours) break;
+        if (entry.precipitationType == "rain" && entry.precipitationProbability > 0.0f) return true;
+        checked++;
+    }
+    return false;
+}