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/platformio.ini

@@ -17,7 +17,9 @@ build_flags =
 ; PSRAM is required for audio ring buffers
     -DBOARD_HAS_PSRAM
 lib_deps =
-; Pinned in later milestones as modules land (I2S mic, SD, BLE, HTTP server).
+; JSON for the on-device REST API (M2). I2S/SD/WiFi/WebServer/mDNS/Preferences are all
+; bundled with the Arduino-ESP32 core, so no other deps are needed yet.
+    bblanchon/ArduinoJson@^7.0
 
 ; --- Build B: ESP32-S3 dev board with PSRAM (N16R8), external I2S mic + SD ---
 [env:esp32s3_dev]