docs: revise Slice 2a Plan 1 to native PostgreSQL (no Docker)

[malinfossum]

Supersedes the Postgres delivery in #29. The only change is how Postgres runs: a native PostgreSQL service locally + a Postgres service container on CI, instead of Docker/Testcontainers. Same two-tier test strategy (SQLite unit tests + real Postgres integration tests), same EF migrations, same ConnectionStrings:WendDb seam.

Rationale: keeping Postgres native avoids the Docker/Hyper-V requirement on Windows dev machines. Henry has agreed to the switch.

What changed vs. the original plan

• App + local integration tests → native Postgres service (no Docker).
• Repo unit tests → unchanged (in-memory SQLite).
• CI → Postgres service container; tests read WEND_TEST_PG.
• Dropped the Testcontainers.PostgreSql dependency.

Stress-test hardening folded in

• Fixture drops leftover wend_test_% databases at run start (persistent-server cleanup).
• Removed a global ClearAllPools(); rely on DROP DATABASE ... WITH (FORCE).
• Documented: keep API tests sequential (avoids template1 contention); the test password default is test-only; CI Host=localhost assumes no job container:.

Docs only — build follows once native Postgres is installed locally.