feat: Grafana copy publishing, dashboard examples, and optional OTel metrics

Implements three observability improvements:

#82 — publish_grafana_copy(): Uses SQLite online backup API (WAL-safe) to
atomically publish a consistent read-only copy beside the target. Adds
--publish-copy option to grafana-schema and snapshot CLI commands.

#83 — examples/grafana/: Minimal working Grafana setup with docker-compose,
provisioning datasource/dashboard YAML, and three dashboard JSON files
(mt5cli-overview, mt5cli-trades, mt5cli-market). All queries use grafana_*
views; no credentials or private paths included.

#84 — mt5cli/telemetry.py: Optional OTel metrics behind mt5cli[otel] extra.
Base install is unaffected. Adds _Mt5Metrics singleton (no-op until
configure_metrics() is called), wraps update_history() and
update_observability() with record_history_update / record_snapshot_update
context managers, and emits account/position gauges from snapshots.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/grafana/provisioning/datasources/mt5cli-sqlite.yaml

@@ -0,0 +1,16 @@
+# Grafana datasource provisioning for mt5cli SQLite.
+#
+# Requires the frser-sqlite-datasource plugin:
+#   grafana-cli plugins install frser-sqlite-datasource
+#
+# Set `path` to the absolute path of your published history.grafana.db file.
+
+apiVersion: 1
+
+datasources:
+  - name: mt5cli-SQLite
+    type: frser-sqlite-datasource
+    access: proxy
+    isDefault: true
+    jsonData:
+      path: /data/mt5cli.db