Phase 1 WP-1: EF Core DbContext with Fluent API mappings for all 26 entities
ScadaLinkDbContext with 10 configuration classes (Fluent API only), initial migration creating 25 tables, environment-aware migration helper (auto-apply dev, validate-only prod), DesignTimeDbContextFactory, optimistic concurrency on DeploymentRecord. 20 tests verify schema, CRUD, relationships, cascades.
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