feat: reporter — HMAC-signed hourly POST with 24-record offline buffer

Fix Arduino String .size() → .length() in hmac.cpp (pre-existing bug surfaced by compilation).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

firmware/src/reporter.h

@@ -0,0 +1,20 @@
+// firmware/src/reporter.h
+#pragma once
+#include <Arduino.h>
+#include "config.h"
+#include "ble_scanner.h"
+
+struct CameraHourlyRecord {
+    uint32_t period_start;
+    uint32_t period_end;
+    int entries;
+    int exits;
+};
+
+static const int    REPORTER_MAX_BUFFER = 24;
+static const char*  REPORTER_API_HOST   = "https://logs.research.bike";
+
+void reporter_submit_camera(const DeviceConfig& cfg, const CameraHourlyRecord& rec);
+void reporter_submit_ble(const DeviceConfig& cfg, const BLEHourlyRecord& rec);
+void reporter_heartbeat(const DeviceConfig& cfg, uint32_t uptime_s, int wifi_rssi);
+void reporter_flush(const DeviceConfig& cfg);