Add live VU-meter readback via ADAU1701 Data Capture Register

Implements GET /api/audio/levels, reading all six 1×RTA level-detector
cells on demand (no background polling, no caching -- only runs when
called, per explicit request this session).

The read mechanism is the ADAU1701's documented Data Capture Register
(address 2074/0x081A, datasheet Rev.0 pp.30,36: write a (program-step,
register-select) pair to configure what the register mirrors, then
read back a 3-byte 5.19 twos-complement value). The per-meter program-
step indices are taken verbatim from SigmaStudio's own compiler output
(Firmware/ADAU1701-Firmware/IC 1_DigiRadioFinale/net_list_out2/
trap.dat), not invented -- the datasheet explicitly says these indices
must come from that compiler-generated file.

Confirmed live: all six points return distinct, plausible dBFS values,
and the output meters track a master-volume change.

Also folds two pieces of live user feedback into the app brief: the
volume slider should reach +12 dB (the firmware's real ceiling), not
stop at 0 dB, and the new levels endpoint is what a "VU meter" UI
section should poll instead of faking one.

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

Software/components/net/src/SetupWebServer.cpp

@@ -971,6 +971,39 @@ esp_err_t audioBeepPostHandler(httpd_req_t* req)
     return httpd_resp_send(req, json.c_str(), json.size());
 }
 
+/**
+ * @brief    audioLevelsGetHandler — serve GET /api/audio/levels as JSON.
+ *
+ * @dname    audioLevelsGetHandler
+ * @param    req  HTTP request handle from esp_http_server.
+ * @return   ESP_OK on success, or an esp_err_t error code.
+ * @pubstate Reads all six 1×RTA level meters live on every call -- no
+ *           background polling, no caching.
+ *
+ * @author   Michele Bigi
+ * @date     2026-08-25
+ */
+esp_err_t audioLevelsGetHandler(httpd_req_t* req)
+{
+    auto* ctx = routeContextFrom(req);
+    if (ctx == nullptr || ctx->audio == nullptr) {
+        httpd_resp_set_status(req, "503 Service Unavailable");
+        return httpd_resp_send(req, nullptr, 0);
+    }
+
+    const auto levels = ctx->audio->readLevels();
+    if (!levels) {
+        const std::string json = core::serializeAudioErrorJson("dsp_failed");
+        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::serializeAudioLevelsJson(*levels);
+    httpd_resp_set_type(req, "application/json");
+    return httpd_resp_send(req, json.c_str(), json.size());
+}
+
 /**
  * @brief    dspParamsGetHandler — serve GET /api/dsp/params as JSON.
  *
@@ -2252,6 +2285,14 @@ std::expected<void, NetError> SetupWebServer::start(
     };
     httpd_register_uri_handler(server_, &audioBeepUri);
 
+    const httpd_uri_t audioLevelsUri = {
+        .uri = "/api/audio/levels",
+        .method = HTTP_GET,
+        .handler = audioLevelsGetHandler,
+        .user_ctx = routeCtx,
+    };
+    httpd_register_uri_handler(server_, &audioLevelsUri);
+
     const httpd_uri_t dspParamsUri = {
         .uri = "/api/dsp/params",
         .method = HTTP_GET,