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>

nextcloud_mcp_server/alembic/versions/20260610_1200_007_add_usage_events.py

@@ -60,7 +60,7 @@ def upgrade() -> None:
         # (json.dumps) on SQLite.
         sa.Column(
             "metadata",
-            postgresql.JSONB if is_pg else sa.Text,
+            postgresql.JSONB() if is_pg else sa.Text,
             nullable=True,
         ),
     )