Slice 2a Plan 1: PostgreSQL + migrations + native test harness

[malinfossum]

Implements Plan 1 (revised) — moves persistence from SQLite to PostgreSQL and adopts EF Core migrations, with no behaviour change. No Docker anywhere: native Postgres locally, a Postgres service container on CI.

Changes (3 commits)

1. Move persistence to PostgreSQL — SQLite→Npgsql in Wend.Core; the app reads ConnectionStrings:WendDb. API integration tests create/drop their own throwaway database per test on the Postgres server (fixture also drops orphans at run start); repository unit tests stay on in-memory SQLite.
2. Adopt EF Core migrations — InitialCreate committed; startup EnsureCreated() → Migrate().
3. CI + docs — CI job gains a postgres:17-alpine service container + WEND_TEST_PG; README documents the native Postgres dev setup.

Verification

• dotnet test → 147 passed, 0 failed, 0 skipped.
• dotnet build -c Release → 0 warnings, 0 errors.
• Migrations round-trip: database update applies InitialCreate cleanly (Boards, Lists, Cards, Labels, CardLabels, ChecklistItems + __EFMigrationsHistory).
• Manual acceptance: created a board, restarted the app, confirmed it persisted in PostgreSQL, then cleaned up.

Plan: docs/plans/2026-07-11-slice2a-postgres-native.md. Establishes the engine + migrations foundation for Plan 2 (Identity + WendUser + per-user scoping).