Add Netlify DB migration tooling

[pablonete]

This continues the Netlify DB rollout by adding the operator tooling needed to apply the schema, copy existing data into DB, and verify parity before enabling DB reads. It also simplifies backend selection so magic-link tokens always use the same storage backend as the rest of the app.

Summary

• Adds server/db-migrate.ts with apply-schema, migrate, and verify commands.
• Adds npm scripts for applying schema, migrating data, and verifying source-to-DB parity.
• Supports migrating from local JSON files or Netlify Blobs via KID_A_MIGRATION_SOURCE=file|blob.
• Verifies stable hashes and counts for conference data, kids, passports, prizes, prize awards, and magic-link tokens.
• Removes KID_A_TOKEN_BACKEND; tokens now follow KID_A_STORE_BACKEND.
• Documents the migration and verification commands.

Notes for review

• This PR does not enable dual writes or DB reads by default.
• The migration script expects NETLIFY_DATABASE_URL to point at the target Netlify DB.
• No UI changes; screenshots are N/A.

Validation

• npm run --silent lint
• npm run --silent build
• npm run --silent build:server
• git --no-pager diff --check

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