Add Spine Events Source Run Summary Index

Why

listSpineCorrelations aggregates spine_events by correlation id and supports source filters. Postgres already carries a source/run summary index for that hot path; SQLite lacked equivalent coverage.

What Changes

• Add the SQLite spine_events(source_kind, source_id, run_id, occurred_at DESC) partial index for source-filtered run aggregation.
• Keep Postgres and SQLite DDL covered by equivalent schema tests.
• Do not change query results, live databases, deployments, or provider runs.

Capabilities

Modified: reference-implementation-architecture

Impact

• Runtime schema: reference-implementation/server/db.js
• Tests: reference-implementation/test/spine-source-boot-backfill.test.js