Fix XID Age graph to show threshold context (#69)

## Summary
- Add fixed Y-axis (0-220M) so the 200M autovacuum threshold is always visible
- Add dashed threshold lines at 150M (yellow warning) and 200M (red danger)
- Update title to clarify the threshold

## Context
The raw XID age naturally trends upward between vacuum freezes, which looked alarming without context. Current values (~143K-216K) are at 0.1% of the threshold - completely healthy.

## Deployment and Testing
- [ ] Sync grafana-config app to feature branch
- [ ] Verify threshold lines appear on PostgreSQL dashboard

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/69

argocd/manifests/grafana-config/dashboards/configmap-postgresql.yaml

@@ -534,13 +534,19 @@ data:
                   "mode": "none"
                 },
                 "thresholdsStyle": {
-                  "mode": "off"
+                  "mode": "dashed"
                 }
               },
               "mappings": [],
+              "max": 220000000,
+              "min": 0,
               "thresholds": {
                 "mode": "absolute",
-                "steps": [{ "color": "green", "value": null }]
+                "steps": [
+                  { "color": "green", "value": null },
+                  { "color": "yellow", "value": 150000000 },
+                  { "color": "red", "value": 200000000 }
+                ]
               },
               "unit": "short"
             }
@@ -566,7 +572,7 @@ data:
               "refId": "A"
             }
           ],
-          "title": "XID Age Over Time",
+          "title": "Tesla XID Age (autovacuum threshold: 200M)",
           "type": "timeseries"
         }
       ],