Add Spine Events Source Run Summary Index

Context

The aggregation hot path builds a GROUP BY query over spine_events, applying stable filters such as source_kind and source_id before hydrating bounded summary rows. Postgres bootstrap and migration DDL already include idx_pg_spine_events_source_run_summary for the source-filtered run summary shape.

Decision

Add the SQLite equivalent index:

CREATE INDEX IF NOT EXISTS idx_spine_events_source_run_summary
  ON spine_events(source_kind, source_id, run_id, occurred_at DESC)
  WHERE run_id IS NOT NULL;

The index is created in the base SQLite schema and the idempotent source-column migration so both fresh and existing SQLite stores converge without data rewrites.

Alternatives

• Add only a broader source index. Rejected because SQLite already has source filtering coverage and the audited gap was the source/run aggregation shape.
• Add live maintenance or provider verification. Rejected for this change; this lane is DDL and local tests only.

Acceptance Checks

• SQLite boot creates idx_spine_events_source_run_summary.
• SQLite and Postgres DDL both contain equivalent source/run summary index coverage.
• Existing aggregation results remain unchanged because only index DDL changes.