Use remix db commands in remaining demos

[mjackson]

The bookstore now uses the database lifecycle commands added in #11640, but Social Auth and Timeboxer still migrated their databases inside the application server. This completes the same conversion across the remaining database-backed demos.

• adds remix db scripts for migration status, migration, and reset in both demos
• exposes the configured seed command in Social Auth
• separates database setup from server startup
• documents the explicit setup and maintenance workflow

The other demos already run their suites through remix test, pass remix doctor, and expose route maps through remix routes without additional configuration.

Before:

pnpm start

After:

pnpm db:reset
pnpm start

Follows #11728.

[github-actions]

Preview Build Available

A preview build has been created for this PR. You can install it using:

pnpm install "remix-run/remix#preview/pr-11729&path:packages/remix"

This preview build will be updated automatically as you push new commits.