feat(influxunifi): add global tags applied to every measurement

Closes #1001. Mirrors the DataDog plugin's global tags feature for
InfluxDB. Per-metric tags take precedence on key collision so
site/device identifiers can never be overwritten by a misconfigured
global. Configurable via TOML/JSON/YAML under influxdb.tags.

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

examples/up.json.example

@@ -21,7 +21,11 @@
      "pass": "unifipoller",
      "db":   "unifi",
      "verify_ssl": false,
-     "interval":   "30s"
+     "interval":   "30s",
+     "tags": {
+       "customer": "abc_corp",
+       "env":      "prod"
+     }
   },
 
   "webserver": {