feat: production-ready firmware with BLE memory management, device_id fixes, and docs

- Reduce debug level to 1 (errors only) for production builds
- Replace BLE pause/resume with full deinit/reinit during HTTP uploads (~25KB freed)
- Add 60s boot report delay for fast post-deploy connectivity verification
- Add device_id to BLE batch and heartbeat request bodies
- Correct API host to http:// (plain HTTP, not HTTPS)
- Add HTTP response logging and CV entry/exit serial logging
- Create root README.md with operator setup and architecture overview
- Update design spec: HMAC format, BLE memory approach, request body shapes, reporting intervals

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

firmware/src/ble_scanner.cpp

@@ -51,6 +51,7 @@ class ScanCallback : public NimBLEAdvertisedDeviceCallbacks {
         std::lock_guard<std::mutex> lock(s_mutex);
         auto it = s_seen.find(hash);
         if (it == s_seen.end()) {
+            Serial.printf("[BLE] new device: %s (rssi %d)\n", hash.c_str(), rssi);
             s_seen[hash] = {rssi, 1};
         } else {
             it->second.rssi_sum += rssi;
@@ -75,9 +76,19 @@ void ble_scanner_start() {
     s_scan->start(0, nullptr, false); // 0 = continuous
 }
 
-void ble_scanner_pause() { if (s_scan) s_scan->stop(); }
+void ble_scanner_pause()  { if (s_scan) s_scan->stop(); }
 void ble_scanner_resume() { if (s_scan) s_scan->start(0, nullptr, false); }
 
+void ble_scanner_deinit() {
+    if (s_scan) s_scan->stop();
+    s_scan = nullptr;
+    NimBLEDevice::deinit(true); // frees NimBLE heap (~25KB)
+}
+
+void ble_scanner_reinit() {
+    ble_scanner_start(); // re-init stack and restart scan
+}
+
 BLEHourlyRecord ble_scanner_collect(uint32_t period_start, uint32_t period_end) {
     // Swap accumulators under lock — minimise time with lock held
     std::map<String, DeviceObs> local_seen;