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/core/include/core/AudioLevels.hpp

@@ -0,0 +1,42 @@
+/**
+ * @file    AudioLevels.hpp
+ * @brief   Snapshot of the six ADAU1701 Data Capture level meters.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-08-25
+ */
+#pragma once
+
+namespace core {
+
+/**
+ * @brief    AudioLevels — one on-demand read of all six 1×RTA meters.
+ *
+ * @dname    AudioLevels
+ * @return   n/a (type)
+ * @pubstate Read-only snapshot; not part of AudioProfile, never persisted.
+ *
+ * Each field is in dBFS, read live from the ADAU1701 Data Capture Register
+ * (address 2074, MAC_out tap) at the program-step index SigmaStudio's
+ * compiler assigned to that 1×RTA cell (see
+ * Firmware/ADAU1701-Firmware/IC 1_DigiRadioFinale/net_list_out2/trap.dat).
+ * Captured only when requested by an API call -- no background polling.
+ *
+ * @author   Michele Bigi
+ * @date     2026-08-25
+ */
+struct AudioLevels {
+    float radioInLeftDb;      ///< 1×RTA1, post-compressor Si4684 L.
+    float radioInRightDb;     ///< 1×RTA2, post-compressor Si4684 R.
+    float bluetoothInLeftDb;  ///< 1×RTA3, ESP32 L.
+    float bluetoothInRightDb; ///< 1×RTA4, ESP32 R.
+    float outputLeftDb;       ///< 1×RTA1_2, post Bass Boost1, L.
+    float outputRightDb;      ///< 1×RTA2_2, post Bass Boost1, R.
+};
+
+} // namespace core

Software/components/core/include/core/AudioProfileJson.hpp

@@ -12,6 +12,7 @@
  */
 #pragma once
 
+#include "core/AudioLevels.hpp"
 #include "core/AudioProfile.hpp"
 #include "core/ParseError.hpp"
 
@@ -104,4 +105,17 @@ namespace core {
 [[nodiscard]] std::expected<bool, ParseError> parseBeepEnabledJson(
     std::string_view json);
 
+/**
+ * @brief    serializeAudioLevelsJson — serialise a level-meter snapshot.
+ *
+ * @dname    serializeAudioLevelsJson
+ * @param    levels  On-demand read from IDsp::readLevels().
+ * @return   JSON object string for GET /api/audio/levels.
+ * @pubstate none
+ *
+ * @author   Michele Bigi
+ * @date     2026-08-25
+ */
+[[nodiscard]] std::string serializeAudioLevelsJson(const AudioLevels& levels);
+
 } // namespace core

Software/components/core/include/core/IDsp.hpp

@@ -13,6 +13,7 @@
 #pragma once
 
 #include "core/ActiveSource.hpp"
+#include "core/AudioLevels.hpp"
 #include "core/AudioProfile.hpp"
 #include "core/DspError.hpp"
 #include "core/EnhanceLevel.hpp"
@@ -183,6 +184,20 @@ public:
      */
     [[nodiscard]] virtual std::expected<void, DspError> writeRawParam(
         unsigned address, float value) = 0;
+
+    /**
+     * @brief    readLevels — on-demand read of all six 1×RTA level meters.
+     *
+     * @dname    readLevels
+     * @return   AudioLevels snapshot, or DspError.
+     * @pubstate Reads live from the ADAU1701; not part of AudioProfile,
+     *           never persisted. Only runs when called, no background
+     *           polling.
+     *
+     * @author   Michele Bigi
+     * @date     2026-08-25
+     */
+    [[nodiscard]] virtual std::expected<AudioLevels, DspError> readLevels() = 0;
 };
 
 } // namespace core

Software/components/core/src/AudioProfileJson.cpp

@@ -338,4 +338,16 @@ std::expected<bool, ParseError> parseBeepEnabledJson(std::string_view json)
     return enabled;
 }
 
+std::string serializeAudioLevelsJson(const AudioLevels& levels)
+{
+    std::ostringstream out;
+    out << "{\"radio_in_left_db\":" << levels.radioInLeftDb
+        << ",\"radio_in_right_db\":" << levels.radioInRightDb
+        << ",\"bluetooth_in_left_db\":" << levels.bluetoothInLeftDb
+        << ",\"bluetooth_in_right_db\":" << levels.bluetoothInRightDb
+        << ",\"output_left_db\":" << levels.outputLeftDb
+        << ",\"output_right_db\":" << levels.outputRightDb << "}";
+    return out.str();
+}
+
 } // namespace core

Software/components/core/test/integration_service_test.cpp

@@ -82,6 +82,12 @@ public:
         return {};
     }
 
+    [[nodiscard]] std::expected<core::AudioLevels, core::DspError>
+    readLevels() override
+    {
+        return core::AudioLevels{};
+    }
+
     [[nodiscard]] std::expected<void, core::DspError> writeRawParam(
         unsigned, float) override
     {