Database migrations

Run from repository root.

• Initial release: 20260908000000_init/migration.sql.
• D1 mirror: backend/migrations-wrangler/0001_init.sql.
• Prisma SQL owns schema history. Keep custom constraints and required seed rows.
• Add future changes as new migrations. Never edit an applied release migration.
• Generate D1 mirrors: npm run migrate:sync.
• Check mirrors: npm run migrate:check.
• Check Prisma schema agreement: npm run prisma:drift.
• Apply locally: npm run local:migrate.
• Remote database writes require explicit deployment authority.