fix: make migrations 034 and 035 idempotent for partial reapply

[mvanhorn]

What does this PR do?

Closes #788

Migrations 034 and 035 weren't safe to rerun against a partially-applied schema. If a previous run's schema commit succeeded but the row write to _emdash_migrations failed (D1 subrequest limit, transient error mid-batch, Worker CPU limit), the next migration run reapplies the schema and crashes with index already exists: SQLITE_ERROR (migration: 034_published_at_index) — which is the exact failure the reporter hit upgrading from 0.1.1 to 0.6.0/0.7.0.

This makes both migrations safe to re-run on a partially-applied schema while preserving accumulated _emdash_404_log data.

• 034_published_at_index.ts: CREATE INDEX → CREATE INDEX IF NOT EXISTS so re-creating an existing per-table index is a no-op.
• 035_bounded_404_log.ts: capture hitsExists once before the column adds. Guard addColumn("hits") and addColumn("last_seen_at") with columnExists checks. On rerun (when hits already exists), skip the dedup rollup UPDATE/DELETE block so live counters and last_seen_at aren't reset back to COUNT(*) = 1 / created_at. Add .ifNotExists() to the new index creates and .ifExists() to the path-index drop (and the symmetric calls in down()).
• dialect-helpers.ts: add columnExists and indexExists helpers. Both branch on isPostgres and follow the same pattern as the existing tableExists. Use pragma_table_info(table) on SQLite, information_schema.columns / pg_indexes on Postgres.
• Added integration test migrations.test.ts that runs all migrations, deletes the rows for 034 and 035 from _emdash_migrations, and re-runs runMigrations against the partially-applied schema. Both names appear in applied, no error is thrown, and the migration table count returns to MIGRATION_COUNT.
• Patch-level changeset.

Type of change

• Bug fix
• Feature (requires maintainer-approved Discussion)
• Refactor (no behavior change)
• Translation
• Documentation
• Performance improvement
• Tests
• Chore (dependencies, CI, tooling)

Checklist

• I have read CONTRIBUTING.md
• pnpm typecheck passes
• pnpm lint passes (no new diagnostics)
• pnpm test passes (packages/core/tests/integration/database/migrations.test.ts 15/15)
• pnpm format has been run
• I have added/updated tests for my changes (if applicable)
• User-visible strings in the admin UI are wrapped for translation (n/a)
• I have added a changeset (if this PR changes a published package)
• New features link to an approved Discussion: n/a (bug fix)

AI-generated code disclosure

• This PR includes AI-generated code

Screenshots / test output

Test Files  1 passed (1)
     Tests  15 passed (15)
  Duration  341ms

The new test exercises the partial-reapply path:

it("should re-run migrations 034 and 035 when schema changes were partially applied", async () => {
    await db.destroy();
    db = await setupTestDatabaseWithCollections();

    await db
        .deleteFrom("_emdash_migrations")
        .where("name", "in", ["034_published_at_index", "035_bounded_404_log"])
        .execute();

    const { applied } = await runMigrations(db);

    expect(applied).toContain("034_published_at_index");
    expect(applied).toContain("035_bounded_404_log");

    const migrations = await db.selectFrom("_emdash_migrations").selectAll().execute();
    expect(migrations).toHaveLength(MIGRATION_COUNT);
});

Out of scope

The middleware behavior the reporter also flagged (auth middleware swallowing the migration error and falling through to a permissionless ghost user) is a separate failure mode in auth.ts and worth its own issue. Per CLAUDE.md's "no drive-by changes" rule, that's not addressed here.

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commit: 1f6e22c

[ascorbic]

/review

[ask-bonk]

LGTM!

github run

[ascorbic]

Thanks!