feat!: add global toggle for SQL trace-context annotation (default off)

[nicolasburtey]

Adds sql_commenter::set_annotation_enabled(bool) — disabled by default (opt-in).

While disabled (the default), annotate_sql is a zero-cost pass-through, so OneTimeExecutor delegates statements untouched: full sqlx prepared-statement reuse for all traffic. Calling set_annotation_enabled(true) once at process startup restores the traceparent annotation introduced in #154.

Motivation: annotated statements are executed persistent(false) — a server-side parse + plan per execution. Under stress-test load (obix outbox checkpoint churn ≈ 250 UPDATE job_executions/s) that cost is significant; this makes annotation an explicit choice and gives operators a config lever to reclaim the cost without recompiling.

lana-bank wires it to tracing.annotate_sql_statements in lana.yml (GaloyMoney/lana-bank#7452).

Tests: annotation_disabled_by_default_and_toggleable covers the default and the toggle; a mutex serializes toggle-sensitive tests since the flag is global. The trace_annotation integration test opts in explicitly.

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Note

Medium Risk
Breaking default: annotation is off unless configured, changing observability and DB execution characteristics for existing deployments that relied on implicit annotation.

Overview
SQL traceparent comments are now opt-in via sql_commenter::set_annotation_enabled / annotation_enabled, backed by a process-wide atomic flag that defaults to disabled.

When the toggle is off, annotate_sql returns the original SQL immediately (Cow::Borrowed), so executors keep full prepared-statement reuse and avoid per-execution parse/plan cost. Enabling it at startup restores sampled-span annotation behavior from the prior feature.

Tests gain a mutex-serialized AnnotationEnabled helper for the global flag, a case for default-off plus toggle, and the Postgres integration test explicitly calls set_annotation_enabled(true) before exercising the end-to-end path.

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