Harden Modbus/Influx paths, add Grafana dashboard

Short or garbled gateway responses are rejected on register count
instead of half-decoding (previously: 'unpack requires a buffer',
OverflowError on garbage scale factors). Scale factors outside
+/-10 and non-finite floats are dropped before they reach line
protocol.

offgrid is written as integer offgrid_state: the existing DB
cannot be migrated (mixed inmem/tsi1 shards block all deletes)
and the old boolean field conflicts with integer writes, which
poisoned the spool - HTTP 400 now drops the batch and logs the
InfluxDB error body instead of re-spooling it forever.

grafana_dashboard.json: fully German dashboard (InfluxQL,
InfluxDB 1.x) with off-grid state, SoC (SoE*SoH/100) and
consumption-source panels; status tiles use integers plus value
mappings because Grafana drops string/boolean fields from
InfluxQL time series.

solaredge_modbus_mqtt.py

@@ -5,6 +5,7 @@ import base64
 import os
 import sys
 import time
+import urllib.error
 import urllib.parse
 import urllib.request
 
@@ -43,6 +44,18 @@ def publish(line, influx):
         send_to_influx(backlog + [line], influx)
         if backlog:
             os.remove(SPOOL_FILE)
+    except urllib.error.HTTPError as err:
+        body = err.read().decode('utf-8', 'replace')[:200]
+        if err.code == 400:
+            # Permanently rejected (malformed line or field type conflict).
+            # Re-spooling would poison every future write, so drop the batch.
+            print('InfluxDB rejected the write, dropping {0} spooled line(s): {1}'.format(len(backlog) + 1, body))
+            if os.path.exists(SPOOL_FILE):
+                os.remove(SPOOL_FILE)
+        else:
+            print('InfluxDB write failed (HTTP {0}), spooling line: {1}'.format(err.code, body or err))
+            with open(SPOOL_FILE, 'a') as f:
+                f.write(line + '\n')
     except Exception as err:
         print('InfluxDB write failed, spooling line: {0}'.format(err))
         with open(SPOOL_FILE, 'a') as f: