Add PUT /api/stream/phone: raw PCM audio push from a phone app

New endpoint accepts chunked, header-less, 16-bit LE stereo PCM @ 48 kHz
in the request body and writes it straight to the shared I2S sink for as
long as the connection stays open. Chosen deliberately unencoded (no
MP3/AAC/Opus decode) to keep this path as simple and low-risk as
possible — the companion app controls the encoding on its side.

Extracted the I2S TX channel that used to be owned outright by
web_radio_stream.cpp into main/esp32_i2s_sink.cpp, shared by both
producers with a simple tryAcquire()/release() exclusivity guard — web
radio streaming and a phone PCM stream would otherwise fight over the
same physical wire. web_radio_stream.cpp now acquires/releases around
each streamWhileEnabled() cycle instead of owning the channel itself.

net::PhoneStreamSink is a plain function-pointer struct (not a class
hierarchy) threaded through NetBootstrap::start() -> SetupWebServer::start()
-> HttpRouteContext, so components/net stays free of I2S driver headers;
main/phone_stream.cpp supplies the concrete functions (bound to
esp32_i2s_sink) and does the int16->ADAU 32-bit-slot conversion, batched
per chunk rather than per sample for the same reason as the web radio
stutter fix (6974095/7e65394 lineage).

Verified by build only — not confirmed live yet (board disconnected this
session); the actual phone app that will exercise this endpoint doesn't
exist yet either.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0178rASQ6ZETPMUamvpoR2KR

Software/main/esp32_i2s_sink.hpp

@@ -0,0 +1,45 @@
+/**
+ * @file    esp32_i2s_sink.hpp
+ * @brief   Shared ESP32 -> ADAU1701 I2S TX channel (web radio / phone stream).
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+
+namespace esp32_i2s_sink {
+
+/**
+ * Open the shared I2S TX channel (ESP32 as I2S slave, ADAU1701 as master)
+ * on first call; a no-op on later calls. Not thread-safe to call
+ * concurrently with itself — call once during boot.
+ * @return true on success.
+ */
+bool open();
+
+/**
+ * Claim exclusive use of the channel for one producer (web radio stream or
+ * a phone PCM stream). Only one producer may hold it at a time, since both
+ * would otherwise fight over the same physical wire.
+ * @return true if acquired, false if another producer already holds it.
+ */
+bool tryAcquire();
+
+/** Release a previously acquired claim. Safe to call even if not held. */
+void release();
+
+/**
+ * Write one frame's worth of already-32-bit-slot-formatted stereo samples
+ * (left, right pairs). Blocks until the DMA accepts the data.
+ * @param samples      Interleaved L,R int32 samples (top-aligned 24-bit).
+ * @param sampleCount  Number of int32 values in samples (2x frame count).
+ * @return true on success.
+ */
+bool writeSamples(const std::int32_t* samples, std::size_t sampleCount);
+
+} // namespace esp32_i2s_sink