docs: Slice 2a Plan 1 — PostgreSQL + migrations + test harness

[malinfossum]

Implementation plan for Slice 2a — Plan 1: the pure infrastructure swap (no auth yet), first of the accounts slice.

• Move persistence from SQLite to PostgreSQL behind the ConnectionStrings:WendDb seam (Npgsql).
• API integration tests move to a throwaway Testcontainers PostgreSQL — one container per run, a fresh database per test; repository unit tests stay on in-memory SQLite. No API test-class changes.
• Adopt EF Core migrations; replace EnsureCreated with Migrate at startup.

Three bite-sized TDD tasks. All 147 tests stay green; no user-visible behaviour change. Plan 2 (Identity + WendUser + Board.OwnerId + per-user scoping) builds on this.

Built from the signed-off spec (PR #28) — merge that first so the plan's spec link resolves.