Track API key last-used dates

[jgpruitt]

Summary

• add day-level api_key.last_used_on metadata and touch_api_key with one-update-per-day semantics
• record usage after successful API-key auth with a process-local cache, lazy 24-hour flush, and best-effort failure handling
• expose lastUsedOn through engine/protocol/RPC/CLI and document the field

Tests

• ./bun test packages/server/middleware/api-key-usage.test.ts
• ./bun test --timeout 30000 packages/database/core/migrate/migrate.integration.test.ts packages/engine/core/core.integration.test.ts packages/engine/core/db.integration.test.ts packages/server/middleware/authenticate-space.integration.test.ts packages/server/middleware/authenticate-user.integration.test.ts packages/server/rpc/user/api-key.integration.test.ts
• ./bun run check
• git diff --check