Node: report every radio heard on a call, not just the last (server-26#304)

P1 of the radio-ID plan. metadata_watcher records each transmission as
[radio, first seen, last seen] from channel rows and call_log grants in both
audio and console modes. Sightings of the same radio under 2 s apart are
merged, because the P0 probe found the ID in only 44% of polls during a
burst. call_end gains `radios: [{radio, start, end}]`.

main.py rebases the offsets into the trimmed audio file's frame, so
wall clock = audio_start_epoch + offset, the mapping C2 already uses. It
also sends the list on /upload (UploadMeta.radios), because C2 starts the
pipeline from /upload before call_end arrives.

srcaddr keeps its meaning (latest talker). Metadata only: no boundary,
timestamp or attribution change (drb-audio-review).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-edge-node/tests/test_upload_spool.py

@@ -228,3 +228,10 @@ async def test_wake_makes_everything_due_now(tmp_path):
     sp.add(_recording(tmp_path), UploadMeta(call_id="c1"), UploadResult(RETRY, status=401))
     sp.wake()
     assert await sp.drain_once() == 1
+
+
+def test_form_carries_the_radio_list_and_omits_it_when_empty():
+    """C2 starts correlation from /upload, so the radio list rides on it (server-26#304)."""
+    radios = json.dumps([{"radio": 555, "start": 0.0, "end": 1.0}])
+    assert upload_spool._form(UploadMeta(call_id="c1", radios=radios))["radios"] == radios
+    assert "radios" not in upload_spool._form(UploadMeta(call_id="c1"))