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>

secondary-sdr-container/app/routers/secondary_controller.py

@@ -1,3 +1,5 @@
+from typing import List, Optional
+
 from fastapi import APIRouter, HTTPException
 from pydantic import BaseModel
 
@@ -11,9 +13,25 @@ class StartBody(BaseModel):
     mode: str  # adsb | ais
 
 
+class StopBody(BaseModel):
+    mode: Optional[str] = None  # omit to stop every decoder
+
+
+class ApplyBody(BaseModel):
+    priority: List[str]  # ordered, e.g. ["adsb", "ais"]
+
+
 def create_secondary_router():
     router = APIRouter()
 
+    @router.post("/apply")
+    async def apply(body: ApplyBody):
+        try:
+            live = decoder_control.apply(body.priority)
+        except ValueError as e:
+            raise HTTPException(status_code=400, detail=str(e))
+        return {"running": live}
+
     @router.post("/start")
     async def start(body: StartBody):
         try:
@@ -21,12 +39,12 @@ def create_secondary_router():
         except ValueError as e:
             raise HTTPException(status_code=400, detail=str(e))
         if not ok:
-            raise HTTPException(status_code=500, detail="Failed to start secondary SDR decoder")
+            raise HTTPException(status_code=409, detail="No free SDR for this decoder")
         return {"status": f"secondary SDR started ({body.mode})"}
 
     @router.post("/stop")
-    async def stop():
-        decoder_control.stop()
+    async def stop(body: Optional[StopBody] = None):
+        decoder_control.stop(body.mode if body else None)
         return {"status": "secondary SDR stopped"}
 
     @router.get("/status")