Secondary SDR priority: every spare SDR runs the next decoder in an ordered list

Replaces the single secondary_sdr_mode with secondary_sdr_priority, e.g.
["adsb", "ais"]. OP25 always keeps its own dongle; the secondary-sdr
container starts decoders top-down until it runs out of free SDRs, so a
3-SDR node runs ADS-B and AIS at once and a 2-SDR node runs the top pick.

- secondary-sdr: one decoder per mode, POST /secondary/apply(priority)
  (no-op when the right prefix is already running), orphaned decoders
  from a uvicorn reload are reaped on start, /status reports sdr_count
  via lsusb (op25's :stable image has none).
- edge-node: one apply path (set_secondary_priority) for the local
  dashboard, a new C2 'set_secondary_priority' MQTT command, and config
  pushes; it never restarts op25. Legacy mode migrates on load. Checkin
  reports priority, what's running, and sdr_count. Uplink forwards
  aircraft and vessels whenever either is present.
- Local dashboard: 'Secondary SDRs' card to enable/reorder/save.

Verified: edge-node pytest 190 passed, flake8 clean.

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

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

@@ -0,0 +1,29 @@
+import asyncio
+from typing import Iterable, List, Optional
+
+from app.internal.config_manager import load_node_config, save_node_config
+from app.internal.logger import logger
+from app.internal.secondary_sdr_client import secondary_sdr_client
+from app.models import normalize_secondary_priority
+
+
+async def set_secondary_priority(priority: Iterable[str]) -> Optional[List[str]]:
+    """Persist and apply the node's secondary SDR priority — one path for the
+    local dashboard, a C2 command and a config push alike. Never touches op25:
+    changing what the spare dongles do must not interrupt P25 recording.
+
+    Returns the decoders now running (None if the container is unreachable).
+    """
+    ordered = normalize_secondary_priority(priority)
+    cfg = load_node_config()
+    cfg.secondary_sdr_priority = ordered
+    cfg.secondary_sdr_mode = ordered[0] if ordered else "none"
+    save_node_config(cfg)
+
+    running = await secondary_sdr_client.apply(ordered)
+    logger.info(f"Secondary SDR priority set to {ordered!r}; running {running!r}")
+
+    # Report straight away so C2's view doesn't wait for the next heartbeat.
+    from app.internal.mqtt_manager import mqtt_manager
+    asyncio.create_task(mqtt_manager.publish_checkin())
+    return running