Implement PostgreSQL persistence baseline, EF Core migrations, and core domain entities

[kalen-releasedgroup]

Summary

Create the primary v1 persistence path and the foundational domain entities needed across the application.

Scope

• Implement PostgreSQL-backed EF Core persistence.
• Create migrations and mappings for the core entities defined in Section 7.2.
• Enforce UTC timestamps, application-assigned IDs, and concurrency tokens where required.

Requirement Links

• Sections 6.2, 7.2 through 7.5, 22.2
• Acceptance Criterion 9

Dependencies

• Sprint 0 scaffolding
• Draft persistence ADR direction

Exit Criteria

• Migrations apply cleanly to PostgreSQL from a fresh database.
• Core entities exist with traceable mappings.
• Persistence design remains portable enough for future SQL Server support.