feat: add @lde/sparql-monitor package

[ddeboer]

Summary

Add a new package for monitoring SPARQL endpoints with periodic checks, storing observations in PostgreSQL.

• MonitorService — orchestrates periodic monitoring with configurable interval
• PostgresObservationStore — stores observations using Drizzle ORM with automatic schema management via pushSchema
• SparqlMonitor — executes SPARQL queries (ASK, SELECT, CONSTRUCT) and measures response time
• Materialized view (latest_observations) for efficient queries of the most recent observation per monitor

Usage

const store = await PostgresObservationStore.create('postgres://...');
const service = new MonitorService({
  store,
  monitors: [{ identifier: 'dbpedia', endpointUrl: new URL('...'), query: 'ASK {...}' }],
  intervalSeconds: 300,
});
service.start();