[llbbl]

Closes #131

Summary

• Add cross-engine CHECK constraints for usernames, password-hash prefixes, message length, and positive timestamps.
• Normalize message authorship to nullable user_id with username backfill, foreign-key behavior, and archive parity.
• Update application joins, exports, commands, seed data, migration safety, and schema documentation.

Testing

• composer test:all
• composer test:integration:websocket
• composer analyse
• composer cs-check
• pnpm check
• pnpm build
• pnpm e2e
• Direct constraint-violation integration coverage on SQLite and MySQL
• Independent PHP and migration reviews passed
• Mutation tests were intentionally not run because they are manual-only.