Add second-SDR fields: secondary_sdr_mode + sdr_count on NodeRecord

Server-side half of node-26#9. NodeRecord gains secondary_sdr_mode
(none|adsb|ais|op25_2) and sdr_count; checkin ingestion stores both,
and PATCH /nodes/{id} accepts and re-pushes secondary_sdr_mode the same
way hardware_preset/ppm_override already work, so it isn't wiped by a
system reassignment (see server-26#111 for the pre-existing bug that
pattern avoids repeating).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

drb-c2-core/app/models.py

@@ -62,6 +62,8 @@ class NodeRecord(BaseModel):
     last_seen: Optional[datetime] = None
     assigned_system_id: Optional[str] = None
     node_type: str = "fixed"  # fixed or portable
+    secondary_sdr_mode: str = "none"  # none | adsb | ais | op25_2 — requires a second physical SDR
+    sdr_count: int = 1  # self-reported by the node's checkin, best-effort
     enforce_override_timeout: bool = True
     is_overridden: bool = False
     override_system_id: Optional[str] = None