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_sdr_client.py

@@ -1,5 +1,5 @@
 import httpx
-from typing import Any, Dict, Optional
+from typing import Any, Dict, List, Optional
 from app.config import settings
 from app.internal.logger import logger
 
@@ -15,6 +15,18 @@ class SecondarySdrClient:
     def __init__(self):
         self.api_url = settings.secondary_sdr_api_url
 
+    async def apply(self, priority: List[str]) -> Optional[List[str]]:
+        """Run decoders down the priority list until SDRs run out. Returns the
+        modes actually running, or None if the container is unreachable."""
+        try:
+            async with httpx.AsyncClient(timeout=30) as client:
+                r = await client.post(f"{self.api_url}/secondary/apply", json={"priority": priority})
+                r.raise_for_status()
+                return r.json().get("running", [])
+        except Exception as e:
+            logger.error(f"Secondary SDR apply (priority={priority!r}) failed: {e}")
+            return None
+
     async def start(self, mode: str) -> bool:
         try:
             async with httpx.AsyncClient(timeout=10) as client: