perf(slm): command center loads from cached status, no device pings

get_live_view fired two device calls on every command-center load:
/measurement-state (sends Measure?) and /live (fresh DOD read) — competing
with the monitor's DOD polling. Both are now redundant: the keepalive monitor
keeps NL43Status fresh (~1.3s) and the live-stream WS handles ongoing updates.

Read the cached /status once instead (no device call); derive is_measuring
from measurement_state. Command center opens instantly without poking the
device. (Relies on monitor_start_time now being in /status.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/routers/slm_dashboard.py

@@ -169,25 +169,18 @@ async def get_live_view(request: Request, unit_id: str, db: Session = Depends(ge
     is_measuring = False
 
     try:
-        async with httpx.AsyncClient(timeout=10.0) as client:
-            # Get measurement state
-            state_response = await client.get(
-                f"{SLMM_BASE_URL}/api/nl43/{unit_id}/measurement-state"
-            )
-            if state_response.status_code == 200:
-                state_data = state_response.json()
-                measurement_state = state_data.get("measurement_state", "Unknown")
-                is_measuring = state_data.get("is_measuring", False)
-
-            # Get live status (measurement_start_time is already stored in SLMM database)
-            status_response = await client.get(
-                f"{SLMM_BASE_URL}/api/nl43/{unit_id}/live"
-            )
-            if status_response.status_code == 200:
-                status_data = status_response.json()
-                current_status = status_data.get("data", {})
+        # Read SLMM's CACHED status (NL43Status) — no device call. The live monitor
+        # keeps it fresh (~1.3s) and the live-stream WS provides ongoing updates, so we
+        # no longer fire Measure? + a fresh DOD read at the device on every command-
+        # center load (which competed with DOD polling for the single connection).
+        async with httpx.AsyncClient(timeout=5.0) as client:
+            r = await client.get(f"{SLMM_BASE_URL}/api/nl43/{unit_id}/status")
+            if r.status_code == 200:
+                current_status = r.json().get("data", {})
+                measurement_state = current_status.get("measurement_state")
+                is_measuring = measurement_state in ("Start", "Measure")
     except Exception as e:
-        logger.error(f"Failed to get status for {unit_id}: {e}")
+        logger.error(f"Failed to get cached status for {unit_id}: {e}")
 
     return templates.TemplateResponse("partials/slm_live_view.html", {
         "request": request,