refactor(usage): address round-1 review on PR #871

- store: add optional `enabled` param to record_usage_event so hot-path
  callers (nc_semantic_search) pass the already-resolved flag instead of
  forcing a second uncached Settings build (ADR-024); falls back to
  get_settings() when None so the store stays self-gating for standalone
  use.
- hooks: thread enabled= through both call sites; bump the outer
  shared()/construction failure log from debug → warning so "metering
  enabled but no billing data" is visible at the default INFO level.
- migration: instantiate postgresql.JSONB() to match the sibling
  TIMESTAMP(timezone=True) column.
- tests: fix the misleading "asyncpg returns JSONB as a JSON string"
  comment; add occurred_at dialect round-trip test and an enabled-param
  short-circuit test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude/scheduled_tasks.lock

@@ -0,0 +1 @@
+{"sessionId":"fdfa73f5-8734-459a-8f23-67e1eeaa804c","pid":13329,"procStart":"23713","acquiredAt":1780838157352}