refactor: deduplicate DB schema in e2e seeders

[sleipi]

Summary

• Replace inline CREATE TABLE statements in both e2e seeders (seed-db.ts, seed-empty-db.ts) with a call to migrate() from src/db/migrations.ts
• Removes ~120 lines of duplicated schema definitions, making migrations.ts the single source of truth
• Seeders now also get indexes automatically, bringing e2e tests closer to production behavior