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/main.cpp

@@ -23,6 +23,8 @@
 #include "si4684/Si4684Tuner.hpp"
 #include "station/StationService.hpp"
 #include "tuner/TunerService.hpp"
+#include "esp32_i2s_sink.hpp"
+#include "phone_stream.hpp"
 #include "web_radio_stream.hpp"
 #include "webradio/WebRadioService.hpp"
 
@@ -197,6 +199,11 @@ extern "C" void app_main()
 
     static webradio::WebRadioService webRadioService(store);
 
+    if (!esp32_i2s_sink::open()) {
+        ESP_LOGW(kTag, "shared I2S sink open failed — web radio and phone "
+                       "streaming will be unavailable");
+    }
+
     auto netResult = net::NetBootstrap::start(
         store,
         tunerService,
@@ -206,6 +213,7 @@ extern "C" void app_main()
         integration,
         otaService,
         webRadioService,
+        phone_stream::sink(),
         hardware::HardwareBootstrap::companionChipStatus(),
         hardware::HardwareBootstrap::deviceIdentity());
     if (!netResult) {