feat: MySQL / MariaDB projection target for `zamsync project`

[Etoile-Bleu]

Summary

Add a MySQL / MariaDB backend to zamsync project so events can be projected to mysql:// targets using INSERT IGNORE INTO.

Context and motivation

Several hospital information systems in low-resource settings (OpenMRS, DHIS2) use MySQL or MariaDB as their primary database. A native mysql:// projection target avoids an extra ETL hop.

Exact task

Follow the same pattern as the planned PostgreSQL target (see #related):

1. Add sqlx with mysql feature to Cargo.toml (if not already present from the PG issue)
2. Create src/cmd/project_mysql.rs with INSERT IGNORE INTO zamsync_events (...) VALUES (...)
3. Table DDL mirrors the SQLite schema with MySQL types (BIGINT UNSIGNED, LONGBLOB for payload)
4. Dispatch in src/cmd/project.rs: if --target starts with mysql://, call project_mysql::run()
5. Integration test gated on TEST_MYSQL_URL env var; skip if unset

Acceptance criteria

• zamsync project <dir> --target mysql://root@localhost/zamsync works end-to-end
• Idempotent: second run inserts 0 rows
• Test skips gracefully when TEST_MYSQL_URL is unset

Estimated effort

~2-4 h -- medium (new flag or filter, integration test)