Introduce Flyway migrations and baseline schema (ref #15)

[sugan0tech]

• Description: Manage schema with Flyway for MySQL and H2.
• Current state: No Flyway dependency; ddl-auto=update used. Entities: User, Space, Map, Element, UserSession, etc.
• Tasks:
  • Add Flyway dependency and disable ddl-auto in prod. Create V1__baseline.sql covering current entities and indexes (including unique constraints and refresh_token_hash).
  • Configure profiles to auto-migrate; use H2-compatible migrations for dev.
  • Document migration flow.
• Acceptance criteria:
  • App boots with Flyway applying migrations on both H2 (dev) and MySQL (build/prod).
• References: pom.xml, entities under features/*, application*.properties.