Pin each SDR service to a dongle by serial; OP25 always opens its SDR by serial

Fixes node-26#11. OP25's generated config said "rtl" (= whichever dongle
enumerates first), so on 2-SDR nodes a decoder could take OP25's dongle
and stop recording. Now:

- sdr_pins: {op25|adsb|ais: serial}, absent = automatic. Every OP25
  config generation (op25_client.generate_config) rewrites the device to
  rtl=<serial>: the pin, else the first dongle's serial, which is what
  "rtl" always opened. Left as "rtl" only for unknown/shared serials.
- secondary-sdr: /secondary/devices lists dongles + serials via librtlsdr
  (works while claimed). apply(priority, pins, reserved) never touches
  OP25's dongle, gives a pinned service only its own dongle, lets a
  higher-priority service take a spare from a lower one, and still runs a
  pinned lower-priority service when the top pick has no dongle.
- sdr_settings.py replaces secondary_priority.py: one apply path for the
  local dashboard, the new set_sdr_config C2 command (set_secondary_priority
  kept as an alias) and config pushes. OP25 restarts only when its own
  dongle changes. Checkin reports sdr_devices, sdr_pins, op25_sdr_serial.
- Local dashboard: 'SDRs' card with an OP25 SDR dropdown and a per-service
  dongle dropdown, duplicate-serial and double-pin warnings.

Verified: edge-node pytest 194 passed; secondary-sdr tests 7 passed;
flake8 clean; page JS passes node --check.

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

drb-edge-node/app/models.py

@@ -38,6 +38,19 @@ def normalize_secondary_priority(items: Iterable[str]) -> List[str]:
     return out
 
 
+# Services that can be bound to a specific dongle by its USB serial.
+SDR_PIN_KEYS = ("op25",) + SECONDARY_SDR_MODES
+
+
+def normalize_sdr_pins(pins: Dict[str, Optional[str]]) -> Dict[str, str]:
+    """Known services only, blank = automatic (dropped). Two services can't
+    share a dongle, so a serial claimed twice raises."""
+    out = {k: str(v).strip() for k, v in (pins or {}).items() if k in SDR_PIN_KEYS and v and str(v).strip()}
+    if len(set(out.values())) != len(out):
+        raise ValueError("Two services can't be pinned to the same SDR.")
+    return out
+
+
 class NodeConfig(BaseModel):
     node_id: str
     node_name: str
@@ -50,6 +63,7 @@ class NodeConfig(BaseModel):
     ppm_override: Optional[float] = None
     node_type: str = "fixed"  # fixed or portable
     secondary_sdr_priority: List[str] = []  # ordered; SDRs beyond op25's run these top-down
+    sdr_pins: Dict[str, str] = {}  # service (op25/adsb/ais) -> dongle serial; absent = automatic
     # Legacy single-mode field (pre-priority). Still written as priority[0] so
     # an older C2 reading checkins sees something sensible; read only to
     # migrate a node_config.json saved before priority existed.