[M2] Prometheus metrics and monitoring

[Fosurero]

Description

Expose operational metrics for production monitoring.

Tasks

• Add prometheus crate dependency
• Metric: blocks_indexed_total (counter)
• Metric: query_latency_seconds (histogram)
• Metric: index_entries_total (gauge)
• Metric: index_build_seconds (histogram)
• HTTP /metrics endpoint
• Example Grafana dashboard JSON

Acceptance Criteria

• Metrics endpoint responds with Prometheus text format
• Dashboard shows indexing throughput and query latency