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/drivers/adau1701/include/adau1701/Adau1701Driver.hpp

@@ -14,6 +14,7 @@
 
 #include "adau1701/Adau1701Error.hpp"
 
+#include "core/AudioLevels.hpp"
 #include "core/AudioProfile.hpp"
 #include "core/EnhanceLevel.hpp"
 #include "core/EqBandIndex.hpp"
@@ -269,6 +270,21 @@ public:
     [[nodiscard]] std::expected<void, Adau1701Error> writeRawParam(
         unsigned address, float value);
 
+    /**
+     * @brief    readLevels — on-demand read of all six 1×RTA level meters.
+     *
+     * @dname    readLevels
+     * @return   AudioLevels snapshot, or Adau1701Error.
+     * @pubstate Reads the ADAU1701 Data Capture Register (address 2074) six
+     *           times in sequence, reconfiguring it for each meter's
+     *           program-step/register-select pair before each read; no
+     *           background polling, only runs when called.
+     *
+     * @author   Michele Bigi
+     * @date     2026-08-25
+     */
+    [[nodiscard]] std::expected<core::AudioLevels, Adau1701Error> readLevels();
+
 private:
     [[nodiscard]] std::expected<void, Adau1701Error> ensureBooted() const;
     [[nodiscard]] std::expected<void, Adau1701Error> safeloadGain(
@@ -277,6 +293,8 @@ private:
         unsigned paramAddr, std::int32_t fixpoint);
     [[nodiscard]] std::expected<void, Adau1701Error> replayProgram(
         const core::DspProgram& program);
+    [[nodiscard]] std::expected<float, Adau1701Error> readCaptureDb(
+        unsigned progCount, unsigned regSel);
 
     Adau1701Pins pins_;
     core::IDspProgramSource& programSource_;

Software/components/drivers/adau1701/include/adau1701/Adau1701Dsp.hpp

@@ -70,6 +70,9 @@ public:
     [[nodiscard]] std::expected<void, core::DspError> writeRawParam(
         unsigned address, float value) override;
 
+    [[nodiscard]] std::expected<core::AudioLevels, core::DspError>
+    readLevels() override;
+
 private:
     [[nodiscard]] static core::DspError mapError(Adau1701Error error) noexcept;