feat(firmware): M2 WiFi + on-device REST API

Adds the WiFi-to-app sync path and the on-device open REST API implementing the
device paths of api/openapi.yaml, plus persistent config in NVS.

What changed:
- firmware/src/config.{h,cpp}: NVS (Preferences) store - stable device id from MAC,
  WiFi SSID/PSK, API bearer token, upload settings. Secrets stay off the SD card.
- firmware/src/net_wifi.{h,cpp}: station join with stored creds; SoftAP provisioning
  fallback when no creds / join fails; mDNS openscribe.local; reconnect in loop().
- firmware/src/api_http.{h,cpp}: synchronous WebServer on :80. Routes: GET /device,
  GET/PUT /device/config, GET /recordings, GET /recordings/{id}, GET
  /recordings/{id}/audio (HTTP Range -> 206 + Content-Range), DELETE /recordings/{id},
  POST /record/start|stop. Bearer-token auth on mutating calls (open when no token).
- firmware/src/storage.{h,cpp}: list/read/delete/count helpers + path builders.
- firmware/src/main.cpp: wire config + WiFi + API into setup/loop; bump fw to 0.2.0.
- firmware/platformio.ini: add bblanchon/ArduinoJson@^7 (only new dep).
- state/: ARCHITECTURE, NOTES, TODO updated; security + NTP follow-ups recorded.

Why:
- This is the WiFi transfer channel and the device half of the "completely open API":
  clients can list, download (resumable), delete and control without any cloud.

Notes:
- Not compiled locally (no PlatformIO on the dev host) or hardware-verified; CI builds
  both board profiles. Security follow-ups logged: API is open when no token is set,
  and the SoftAP uses a fixed default passphrase (make user-set at M4/BLE).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

firmware/src/config.h

@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: GPL-3.0-only
+// Persistent device configuration in NVS (Preferences). Holds the device id, WiFi
+// credentials, the API bearer token, and upload settings (upload is used from M3).
+// Secrets live here, never on the SD card in clear.
+#pragma once
+
+#include <Arduino.h>
+
+namespace config {
+
+// Open NVS and ensure a stable device id exists (derived from the MAC on first boot).
+bool begin();
+
+String deviceId();
+
+String wifiSsid();
+String wifiPsk();
+void setWifi(const String &ssid, const String &psk);
+
+// Bearer token for mutating API calls. Empty = auth disabled (open on a trusted LAN).
+String apiToken();
+void setApiToken(const String &token);
+
+// Upload settings (consumed in M3). Getters return stored values or sensible defaults.
+String uploadTarget();  // "none" | "s3" | "webdav"
+void setUploadTarget(const String &target);
+
+}  // namespace config