Add MySQL support via Drizzle ORM

[PAMulligan]

Description

Add MySQL as a supported database provider. Drizzle ORM has full MySQL support via `mysql2` and `drizzle-orm/mysql-core`. Many enterprises and existing projects use MySQL, so supporting it broadens Nerva's audience.

Why

MySQL is the world's most popular open-source database. Excluding it limits Nerva's adoption in organizations with existing MySQL infrastructure. PlanetScale (MySQL-compatible) is also a popular serverless database option.

Acceptance Criteria

• Add `--mysql` flag to `setup-project.sh`
• Generate MySQL-compatible Drizzle schema (`mysqlTable` instead of `pgTable`)
• Use `mysql2` driver instead of `postgres`
• Generate appropriate `drizzle.config.ts` for MySQL
• Update Docker templates with MySQL service option
• Update `pipeline.config.json` to support `"mysql"` provider
• Update `database-design` skill to handle MySQL column types and differences
• Document MySQL-specific considerations (no array columns, different JSON syntax, etc.)
• CI passes

Notes

PostgreSQL remains the recommended default. MySQL support is for teams with existing MySQL infrastructure.

[github-actions]

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