fix(slm): show real cache freshness in device list + draw L1/L10 on card chart

1. "No recent check-in" was always shown because the row's last-check text read
   unit.slm_last_check (a Terra-View roster field the monitor never updates),
   while the live freshness lives in SLMM's cached NL43Status.last_seen. Carry
   that last_seen onto the unit (unit.cache_last_seen) and display it (falling
   back to slm_last_check). Also treat "Measure" as Measuring in the badge, to
   match the panel and the cache's MEASURING_STATES.

2. The dashboard card chart only had Lp + Leq datasets, so L1/L10 never drew even
   though the cards showed them. Add L1 (purple) and L10 (orange) datasets and
   feed ln1/ln2 in both the /history backfill and the live /monitor frames.
   Percentiles parse via numOrNull so a missing "-.-" leaves a gap (spanGaps)
   instead of dropping the line to 0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/routers/slm_dashboard.py

@@ -94,6 +94,7 @@ async def get_slm_units(
         # Legacy default from the roster field; refined from SLMM's cached status below.
         unit.is_recent = bool(unit.slm_last_check and unit.slm_last_check > one_hour_ago)
         unit.measurement_state = None
+        unit.cache_last_seen = None  # SLMM cache last_seen (real monitoring freshness)
 
     if include_measurement:
         # SLMM's /roster carries each unit's CACHED status (last_seen,
@@ -124,6 +125,7 @@ async def get_slm_units(
                 try:
                     ls = datetime.fromisoformat(last_seen.replace("Z", ""))
                     unit.is_recent = ls > recent_cutoff
+                    unit.cache_last_seen = ls  # the real freshness the monitor updates
                 except Exception:
                     pass