feat: mark Modbus connection loss in InfluxDB

SolarEdge firmware is known to silently drop the Modbus TCP listener
(port refuses connections while still shown as enabled). Write a
connection_state=1 point on first failed poll and 0 on recovery so the
outage is visible in Grafana instead of just a data gap.

ConnectionException is re-raised from read() - it is fatal for the
whole cycle, not a flaky single block. Marker points carry no tags
(registers unreadable during outage), so the tile query skips the
serial filter.

Also includes threshold tweaks on the power stat tiles.

grafana_dashboard.json

@@ -37,6 +37,19 @@
       },
       "fieldConfig": {
         "defaults": {
+          "thresholds": {
+            "mode": "absolute",
+            "steps": [
+              {
+                "color": "text",
+                "value": null
+              },
+              {
+                "color": "green",
+                "value": 0
+              }
+            ]
+          },
           "unit": "watt"
         },
         "overrides": []
@@ -82,6 +95,19 @@
       },
       "fieldConfig": {
         "defaults": {
+          "thresholds": {
+            "mode": "absolute",
+            "steps": [
+              {
+                "color": "red",
+                "value": null
+              },
+              {
+                "color": "green",
+                "value": 0
+              }
+            ]
+          },
           "unit": "watt"
         },
         "overrides": []
@@ -117,7 +143,7 @@
           "resultFormat": "time_series"
         }
       ],
-      "title": "Netz aktuell (+ Bezug / - Einspeisung)",
+      "title": "Netz aktuell (+ Einspeisung / - Bezug)",
       "type": "stat"
     },
     {
@@ -127,6 +153,19 @@
       },
       "fieldConfig": {
         "defaults": {
+          "thresholds": {
+            "mode": "absolute",
+            "steps": [
+              {
+                "color": "green",
+                "value": null
+              },
+              {
+                "color": "red",
+                "value": 0
+              }
+            ]
+          },
           "unit": "watt"
         },
         "overrides": []
@@ -2818,6 +2857,80 @@
       ],
       "title": "Temperaturen",
       "type": "timeseries"
+    },
+    {
+      "datasource": {
+        "type": "influxdb",
+        "uid": "${DS_INFLUXDB}"
+      },
+      "fieldConfig": {
+        "defaults": {
+          "mappings": [
+            {
+              "type": "value",
+              "options": {
+                "0": {
+                  "text": "OK",
+                  "color": "green",
+                  "index": 0
+                },
+                "1": {
+                  "text": "Verbindung verloren",
+                  "color": "red",
+                  "index": 1
+                }
+              }
+            }
+          ],
+          "thresholds": {
+            "mode": "absolute",
+            "steps": [
+              {
+                "color": "green",
+                "value": null
+              },
+              {
+                "color": "red",
+                "value": 1
+              }
+            ]
+          }
+        },
+        "overrides": []
+      },
+      "gridPos": {
+        "h": 4,
+        "w": 4,
+        "x": 0,
+        "y": 94
+      },
+      "id": 109,
+      "options": {
+        "colorMode": "background",
+        "graphMode": "none",
+        "textMode": "value",
+        "reduceOptions": {
+          "calcs": [
+            "lastNotNull"
+          ],
+          "fields": "",
+          "values": false
+        }
+      },
+      "targets": [
+        {
+          "datasource": {
+            "type": "influxdb",
+            "uid": "${DS_INFLUXDB}"
+          },
+          "query": "SELECT last(\"connection_state\") FROM \"$measurement\" WHERE $timeFilter",
+          "refId": "A",
+          "rawQuery": true,
+          "resultFormat": "time_series"
+        }
+      ],
+      "title": "Modbus-Verbindung",
+      "type": "stat"
     }
   ],
   "refresh": "1m",