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>

state/NOTES.md

@@ -52,6 +52,27 @@ Per component (see each component's README for detail):
   then. See the follow-ups in TODO.md.
 - Not yet compiled locally (no PlatformIO on the dev host) - CI builds both profiles.
 
+## Firmware specifics (M2)
+
+- REST API served by the core-bundled synchronous `WebServer` on port 80. Path params via
+  `server.on("/.../{}" , ...)` + `server.pathArg(0)`. `collectHeaders` is required to read
+  `Authorization` and `Range`. Register `/recordings/{}/audio` before `/recordings/{}`.
+- Audio download implements HTTP Range (206 + Content-Range) by seeking the file and
+  streaming with `sendContent`; full GET sends `Accept-Ranges: bytes`.
+- Only new dep is `bblanchon/ArduinoJson@^7` (request parse + JSON responses). WiFi,
+  WebServer, ESPmDNS, Preferences, SD are all core-bundled.
+- WiFi: station mode with stored creds; on no-creds/failure it opens a SoftAP
+  `OpenScribe-<macsuffix>` (pass `openscribe`) so the API is reachable to provision WiFi.
+  mDNS name is `openscribe.local`.
+- Config/secrets live in NVS (`Preferences` namespace `openscribe`), never on the SD card.
+
+## Security follow-ups (before any real deployment)
+
+- API auth is open when no token is set (convenient on a trusted LAN). Set an `api_token`
+  via `PUT /device/config` to require `Authorization: Bearer` on mutating calls; consider
+  requiring it for reads too. Tracked in TODO.
+- SoftAP uses a fixed default passphrase; make it user-set during provisioning (M4/BLE).
+
 ## Dead ends
 
 - (none yet)