Add Spine Events Source Run Summary Index

1. Schema

• Add SQLite base-schema source/run summary index.
• Add SQLite idempotent migration creation for existing stores.
• Confirm Postgres already carries equivalent source/run summary DDL.

2. Tests

• Add SQLite schema assertion for the new index.
• Add static DDL parity assertion for SQLite and Postgres text.

3. Acceptance Checks

• Run openspec validate add-spine-events-source-run-summary-index --strict.
• Run focused schema tests.
• Run focused typecheck as feasible.