Fix ADAU1701 SerialInputRegister IBP override causing persistent hiss

The 2026-08-16 IBP=1 override (commit 6974095) was applied alongside a
separate, simultaneous fix to Si4684's PIN_CONFIG_ENABLE and credited
with turning static into audible music. SerialInputRegister (0x081F) is
a single register shared by every SDATA_INx pin on the ADAU1701, so the
override also applied to the ESP32 streaming input, not just Si4684's.
Live A/B testing today (DAB, FM, and ESP32 web-radio, all isolated via
mixer gain, with and without ADAU1701 DSP bypass, with and without BT
A2DP codec changes) narrowed the hiss to this one shared register.
Removing the override and leaving IBP at its compiled default (0x00)
resolved the hiss on both the Si4684 and ESP32 paths, confirmed by ear.

Also:
- Add a runtime HTTP API (GET/POST /api/bluetooth/a2dp-codec) to change
  the BT1035 A2DP codec bitmask without reflashing, used to rule out
  AAC/SBC codec choice as a contributing cause.
- Extend the ADAU1701 boot-time EQ diagnostic to read back all 6 bands
  (previously band 0 only) using the correct 5.23 fixed-point format.
- Note EEPROM persistence for ANTCAP/crystal calibration as TODO.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Software/components/net/src/SetupWebServer.cpp

@@ -1759,6 +1759,63 @@ esp_err_t bluetoothAutoReconnectPostHandler(httpd_req_t* req)
     return httpd_resp_send(req, "{\"status\":\"saved\"}", 18);
 }
 
+esp_err_t bluetoothA2dpCodecConfigPostHandler(httpd_req_t* req)
+{
+    auto* ctx = routeContextFrom(req);
+    if (ctx == nullptr || ctx->bluetooth == nullptr) {
+        httpd_resp_set_status(req, "503 Service Unavailable");
+        return httpd_resp_send(req, nullptr, 0);
+    }
+
+    std::array<char, 128> body{};
+    (void)readRequestBody(req, body);
+    const auto mask = core::parseBluetoothA2dpCodecConfigJson(
+        std::string_view(body.data()));
+    if (!mask) {
+        const std::string json =
+            core::serializeBluetoothErrorJson(parseErrorToken(mask.error()));
+        httpd_resp_set_status(req, "400 Bad Request");
+        httpd_resp_set_type(req, "application/json");
+        return httpd_resp_send(req, json.c_str(), json.size());
+    }
+
+    if (auto result = ctx->bluetooth->setA2dpCodecConfig(*mask); !result) {
+        const std::string json =
+            core::serializeBluetoothErrorJson(bt1035ErrorToken(result.error()));
+        httpd_resp_set_status(req, "500 Internal Server Error");
+        httpd_resp_set_type(req, "application/json");
+        return httpd_resp_send(req, json.c_str(), json.size());
+    }
+
+    ESP_LOGI(kTag, "A2DP codec config set to bitmask %u — reconnect the "
+                   "peer for it to take effect",
+             static_cast<unsigned>(*mask));
+    httpd_resp_set_type(req, "application/json");
+    return httpd_resp_send(req, "{\"status\":\"saved\"}", 18);
+}
+
+esp_err_t bluetoothA2dpCodecGetHandler(httpd_req_t* req)
+{
+    auto* ctx = routeContextFrom(req);
+    if (ctx == nullptr || ctx->bluetooth == nullptr) {
+        httpd_resp_set_status(req, "503 Service Unavailable");
+        return httpd_resp_send(req, nullptr, 0);
+    }
+
+    const auto codec = ctx->bluetooth->queryA2dpCodec();
+    if (!codec) {
+        const std::string json =
+            core::serializeBluetoothErrorJson(bt1035ErrorToken(codec.error()));
+        httpd_resp_set_status(req, "500 Internal Server Error");
+        httpd_resp_set_type(req, "application/json");
+        return httpd_resp_send(req, json.c_str(), json.size());
+    }
+
+    const std::string json = core::serializeBluetoothA2dpCodecJson(*codec);
+    httpd_resp_set_type(req, "application/json");
+    return httpd_resp_send(req, json.c_str(), json.size());
+}
+
 esp_err_t stationsGetHandler(httpd_req_t* req)
 {
     auto* ctx = routeContextFrom(req);
@@ -2334,6 +2391,22 @@ std::expected<void, NetError> SetupWebServer::start(
     };
     httpd_register_uri_handler(server_, &bluetoothAutoReconnectUri);
 
+    const httpd_uri_t bluetoothA2dpCodecConfigUri = {
+        .uri = "/api/bluetooth/a2dp-codec",
+        .method = HTTP_POST,
+        .handler = bluetoothA2dpCodecConfigPostHandler,
+        .user_ctx = routeCtx,
+    };
+    httpd_register_uri_handler(server_, &bluetoothA2dpCodecConfigUri);
+
+    const httpd_uri_t bluetoothA2dpCodecGetUri = {
+        .uri = "/api/bluetooth/a2dp-codec",
+        .method = HTTP_GET,
+        .handler = bluetoothA2dpCodecGetHandler,
+        .user_ctx = routeCtx,
+    };
+    httpd_register_uri_handler(server_, &bluetoothA2dpCodecGetUri);
+
     const httpd_uri_t stationsGetUri = {
         .uri = "/api/stations",
         .method = HTTP_GET,