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/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