Fix Overpass health + cache-hit stat panels: vector(0) not clamp_min

The original formulas used clamp_min(denominator, 1) to avoid divide-
by-zero, but that inflates the denominator when request rate is < 1/s.
Real example from prod: ~0.04 req/s, all successful, displayed as
1.02% health (red) instead of ~100%.

Switch to (numerator or vector(0)) / denominator:
- Missing "hit" series (no cache hits yet) → numerator resolves to 0
  instead of empty set, so the stat shows 0% instead of "No data".
- Zero traffic (empty denominator) → whole expr is empty, Grafana
  renders "No data", which is the correct behaviour for a health
  stat when nothing's happening.
- Non-zero low traffic → ratio is true ratio, not artificially
  depressed by the clamp.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

infrastructure/grafana/dashboards/planner.json

@@ -109,7 +109,7 @@
       },
       "targets": [
         {
-          "expr": "sum(rate(overpass_upstream_requests_total{status=~\"2..\"}[5m])) / clamp_min(sum(rate(overpass_upstream_requests_total[5m])), 1)",
+          "expr": "(sum(rate(overpass_upstream_requests_total{status=~\"2..\"}[5m])) or vector(0)) / sum(rate(overpass_upstream_requests_total[5m]))",
           "legendFormat": "success"
         }
       ],
@@ -140,7 +140,7 @@
       },
       "targets": [
         {
-          "expr": "sum(rate(overpass_cache_events_total{result=\"hit\"}[5m])) / clamp_min(sum(rate(overpass_cache_events_total[5m])), 1)",
+          "expr": "(sum(rate(overpass_cache_events_total{result=\"hit\"}[5m])) or vector(0)) / sum(rate(overpass_cache_events_total[5m]))",
           "legendFormat": "hit ratio"
         }
       ],