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/storage.h

@@ -36,4 +36,21 @@ class WavWriter {
 // Write a sidecar JSON metadata file next to a recording.
 bool writeMetadata(const String &path, const String &json);
 
+// Path helpers for a recording id.
+String recWavPath(const String &id);
+String recMetaPath(const String &id);
+
+// Read a whole (small) file into a String; empty String if it does not exist.
+String readFile(const String &path);
+
+// Delete a recording's audio + metadata. Returns true if anything was removed.
+bool removeRecording(const String &id);
+
+// Count recordings (by .wav files) currently on the card.
+uint32_t countRecordings();
+
+// Build the RecordingPage JSON by concatenating every sidecar metadata object. This is
+// the body for GET /recordings and matches api/openapi.yaml.
+String listRecordingsJson();
+
 }  // namespace storage