fix: add indexes for watcher and retention queries

[Hakimi674]

Overview

This PR adds supporting database indexes for the watcher and retention/cleanup queries that filter by status and timestamps. It ensures migrations are repeatable and safe for both fresh and already-migrated SQLite and PostgreSQL databases, keeping query cost predictable as transaction and event tables grow.

Related Issue

Changes

🗄️ Database Indexes

• [MODIFY] src/runtime/database/sql-database-adapter.ts
  • Add index creation for pending status plus created/expiry timestamps used by the current watcher and cleanup queries.
  • Make index migrations idempotent so they apply safely on both fresh and already-migrated databases.
  • Keep behavior compatible across SQLite and PostgreSQL.

✅ Tests

• [MODIFY] tests/runtime/database/sql-database-adapter.test.ts
  • Assert the indexes are created and that migration is repeatable.
• [MODIFY] tests/runtime/sql-adapter-watcher-tasks.test.ts
  • Confirm watcher query results are unchanged with the new indexes present.
• [MODIFY] tests/runtime/sql-adapter-cleanup.test.ts
  • Confirm retention/cleanup query results are unchanged with the new indexes present.

Verification Results

bun test
✅ passing

Acceptance Criteria	Status
Existing query results remain unchanged	✅ Watcher and cleanup tests assert identical results
A fresh and already-migrated database receives the indexes safely	✅ Idempotent migration covered by schema assertions
Indexes added for pending status plus created/expiry timestamps	✅ Added in sql-database-adapter.ts
Migrations repeatable for SQLite and PostgreSQL	✅ Idempotent index creation

Closes #599

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