Observability: observe hook, span attributes, parameter redaction #77

Signal

PR #33 added spans for every statement. We want more:

• An observe hook with duration, row count, error tag, dialect, tenant scope and route.
• Span attributes in OpenTelemetry semantic conventions (db.system, db.operation.name, db.collection.name).
• Parameter values redacted by default. A column codec marks a value as sensitive (Schema.Redacted), and the value never reaches a span or a log.
• One span per transaction or batch, as an option. Maple does this today with a statement collector.

Why (MyRemotely)

• We store phone numbers, emails, door codes and API key hashes. Our code uses Redacted in 178 places.
• Our error tracker and TestLog read spans. One leaked door code in a span is a security incident.

Part of #52: the CAFE-stack roadmap signal from MyRemotely. This shows the direction we want, and it is not a demand. We can send a PR if you accept the idea.