edge-node: Trunk Recorder backend behind RECORDER_BACKEND (node-26#45)

RECORDER_BACKEND=trunk-recorder (default op25, unchanged):
- tr_config: TR config.json + talkgroup/channel CSVs from the C2 system
  config (P25 trunked, P25 conventional, analog with squelch).
- tr_calls: TR call JSON -> MQTT call_start/call_end + /upload fields
  (talkgroup, start/stop, freq, units from srcList, emergency, encrypted).
- trunk_recorder: inbox ingest through the #44 spool with deterministic
  call_ids, supervisor requests, live audio via simplestream -> pacat ->
  drb_sink for Discord (one talkgroup at a time).
- radio_backend: start/stop dispatch so OP25 never grabs the SDR, and the
  OP25 watcher/PulseAudio recorder stay off (no double uploads).
- health/dashboard/checkin show the active backend and TR health.

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

drb-edge-node/app/internal/radio_backend.py

@@ -0,0 +1,55 @@
+"""
+Start/stop "the radio" without every caller knowing which recorder backend is
+active (node-26#45). Every path that used to call op25_client.start/stop — C2
+config pushes, the op25_restart command, the dashboard, SDR re-pinning — goes
+through here.
+
+With RECORDER_BACKEND=trunk-recorder, OP25 is never started: both open the
+same RTL-SDR, and OP25 holding it would starve Trunk Recorder. start() becomes
+a restart request to trunk-recorder/run.sh instead.
+"""
+import time
+from typing import Optional
+
+from app.internal import trunk_recorder as tr
+from app.internal.logger import logger
+from app.internal.op25_client import op25_client
+
+
+def name() -> str:
+    return "Trunk Recorder" if tr.enabled() else "OP25"
+
+
+async def start(wait: bool = True) -> bool:
+    return await start_with_reason(wait) is None
+
+
+async def start_with_reason(wait: bool = True) -> Optional[str]:
+    """None on success, else a human-readable reason.
+
+    For Trunk Recorder, `wait` blocks until run.sh reports TR restarted (up
+    to trunk_recorder.RESTART_WAIT_SECONDS), like op25_client.start returns
+    only once OP25 has started — callers hand the spare dongles out next.
+    """
+    if not tr.enabled():
+        return await op25_client.start_with_reason()
+    if tr.trunk_recorder.config_error:
+        return tr.trunk_recorder.config_error
+    requested = time.time()
+    if not tr.trunk_recorder.request_restart():
+        return f"Couldn't write to {tr.config_dir()} to restart Trunk Recorder."
+    if not wait:
+        return None
+    err = await tr.trunk_recorder.wait_started_since(requested)
+    if err:
+        logger.error(err)
+    return err
+
+
+async def stop() -> bool:
+    if not tr.enabled():
+        return await op25_client.stop()
+    ok = tr.trunk_recorder.request_stop()
+    if ok:
+        logger.info("Trunk Recorder stop requested.")
+    return ok