refactor(usage): address round-2 review on PR #871

- remove accidentally-committed .claude/scheduled_tasks.lock (Claude Code
  runtime artifact swept in by `git add -A`) and gitignore it; the rest
  of .claude/ stays tracked.
- store: cache UsageEventStore.shared() as a process-wide instance so the
  hot search path doesn't allocate a fresh wrapper per metered query (the
  wrapper is stateless beyond its storage handle).
- hooks: pass enabled=True directly (the outer guard already confirmed
  the flag) instead of re-reading settings.usage_metering_enabled.
- migration: document the no-TTL retention design (control-plane rollup
  owns the lifecycle; the data plane only appends).
- tests: assert the best-effort error path logs at WARNING (observability
  contract).

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

nextcloud_mcp_server/vector/processor.py

@@ -839,9 +839,10 @@ async def _index_document(
                             "user_id": doc_task.user_id,
                             "total_chars": total_chars,
                         },
-                        # Pass the already-resolved flag so the store doesn't
-                        # rebuild Settings here (ADR-024).
-                        enabled=settings.usage_metering_enabled,
+                        # The outer guard already confirmed the flag, so pass
+                        # enabled=True directly — the store then skips a second
+                        # uncached Settings build here (ADR-024).
+                        enabled=True,
                     )
                 except Exception:
                     # Reached only when shared()/store construction itself