Add full FM band scan for building a station/channel list

New TunerService::scanFullFmBand(): tunes to the band bottom then seeks up
repeatedly (reusing the existing hardware-seek + RDS-name-poll machinery
from scanForStation()) until the sweep wraps back around, collecting every
station that clears the existing scan RSSI/SNR thresholds. Returns the
list without touching saved presets or leaving the tuner in any particular
place — callers decide what to do with the results.

New POST /api/tuner/scan/full endpoint (core::TunerFmScannedStation DTO,
serializeTunerFmBandScanJson). Blocks for the whole sweep like the existing
/api/tuner/scan.

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

Software/components/core/test/tuner_json_test.cpp

@@ -139,6 +139,29 @@ namespace {
     return EXIT_SUCCESS;
 }
 
+[[nodiscard]] int runTunerFmBandScanSerialiseTest()
+{
+    core::TunerFmScannedStation withName{
+        *core::FrequencyKHz::tryFromKhz(98300U), 12, 13,
+        core::BroadcastLabel::tryFromChipBytes("RADIO 1")};
+    core::TunerFmScannedStation withoutName{
+        *core::FrequencyKHz::tryFromKhz(105500U), -4, -2, std::nullopt};
+
+    const std::string json =
+        core::serializeTunerFmBandScanJson({withName, withoutName});
+    if (!expectEqual(
+            json,
+            R"({"stations":[{"frequency_khz":98300,"rssi_dbuv":12,"snr_db":13,"station_name":"RADIO 1"},{"frequency_khz":105500,"rssi_dbuv":-4,"snr_db":-2}]})")) {
+        return EXIT_FAILURE;
+    }
+
+    const std::string emptyJson = core::serializeTunerFmBandScanJson({});
+    if (!expectEqual(emptyJson, R"({"stations":[]})")) {
+        return EXIT_FAILURE;
+    }
+    return EXIT_SUCCESS;
+}
+
 [[nodiscard]] int runTunerStatusMetadataSerialiseTest()
 {
     core::TunerStatus status = {};
@@ -240,5 +263,8 @@ int main()
     if (runTunerSeekParseTest() != EXIT_SUCCESS) {
         return EXIT_FAILURE;
     }
+    if (runTunerFmBandScanSerialiseTest() != EXIT_SUCCESS) {
+        return EXIT_FAILURE;
+    }
     return EXIT_SUCCESS;
 }