Secondary SDR panel: never present an unreported SDR count or run state as fact

QA (drb-qa-review) blockers: sdr_count defaulted to 1 for nodes that
never sent it, so the panel claimed 'reports 1 SDR (0 spare)'. It is now
None until reported, and the count is only quoted alongside a real
secondary_sdr_running report. Rows read 'Not reported' instead of
'Waiting for SDR' when the node hasn't said what's running (closes #187).

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

drb-c2-core/app/internal/mqtt_handler.py

@@ -114,7 +114,7 @@ class MQTTHandler:
                 "secondary_sdr_mode": payload.get("secondary_sdr_mode", "none"),
                 "secondary_sdr_priority": payload.get("secondary_sdr_priority", []),
                 "secondary_sdr_running": payload.get("secondary_sdr_running"),
-                "sdr_count": payload.get("sdr_count", 1),
+                "sdr_count": payload.get("sdr_count"),  # None until reported, never a guessed 1
                 "enforce_override_timeout": payload.get("enforce_override_timeout", True),
                 "is_overridden": False,
                 "override_system_id": None,

drb-c2-core/app/models.py

@@ -68,7 +68,7 @@ class NodeRecord(BaseModel):
     # checkin, which is the source of truth; set via PATCH /nodes/{id}.
     secondary_sdr_priority: List[str] = []
     secondary_sdr_running: Optional[List[str]] = None  # what the node reports actually running
-    sdr_count: int = 1  # self-reported by the node's checkin, best-effort
+    sdr_count: Optional[int] = None  # self-reported by the node's checkin; None = never reported
     enforce_override_timeout: bool = True
     is_overridden: bool = False
     override_system_id: Optional[str] = None