Correct and verify database setup instructions

[ejones23]

Summary

• add a production-safe rollout checklist for the ballot management-token migration
• document backup, conflict preflight, schema verification, deployment ordering, canary checks, monitoring, and conservative rollback
• add ballot_management_tokens to the expected fresh-install table list and schema reference
• replace the nonexistent Schema.sql and Docker Compose references with the maintained production schema and ordered migrations
• correct the preview command and Vite port in developer/startup guidance

Verification

• reapplied the migration to local MySQL to verify idempotence
• confirmed InnoDB, the six expected columns, primary/unique/ballot indexes, and zero rows
• focused V2BallotTest: 6 tests / 48 assertions
• full suite: 191 Vitest tests; 231 PHPUnit tests / 587 assertions
• bash syntax check for start-local.sh
• stale setup-reference scan and git diff --check

Context

Follow-up to merged PR #81. The full fresh-install schema already contained the table and the existing-database section already named the migration; this closes the missing verification and rollback guidance and removes stale alternate setup paths.

[DavidMoritz]

Great housekeeping! Fixing the stale port numbers, npm command syntax, and nonexistent file references in AGENTS.md is really valuable — accurate docs save everyone time. The rollout checklist for the management-token migration is thorough and operationally useful, covering backup, conflict detection, deploy ordering, and verification. Thanks for catching all of these, Emmanuel!