feat: Sprint 1 remainder — Postgres store, Docker client, warm pool, sessions route

- PostgresUserStore (Queryable) + migration + env-gated wiring
- DockerodeClient behind injected DockerEngine
- ImageManager, WarmPool, Dockerfile.wordpress, docker-compose
- POST /sessions spins up sandbox (503 when unconfigured)
- ADRs 0014-0015; 68 unit tests green

docs/decisions/0014-postgres-queryable.md

@@ -0,0 +1,26 @@
+# 14. Postgres user store via a Queryable boundary; schema in SQL migrations
+
+- **Status:** Accepted
+- **Date:** 2026-08-15
+
+## Context
+
+ADR 0012 deferred Postgres behind the `UserStore` interface. Sprint 1 now ships the real implementation. Postgres is not available in the dev environment, so the store had to be testable without a database.
+
+## Decision
+
+`PostgresUserStore` uses `node-postgres` (`pg`) but depends on a minimal `Queryable` interface (`query(text, values)`) instead of `pg.Pool` directly. `UserStore` methods became async. The `users` table lives in `api/migrations/001_init.sql`. `index.ts` selects Postgres when `DATABASE_URL` is set, else the in-memory store.
+
+### Options considered
+
+- ORM (Prisma/Drizzle) with migrations.
+- Raw `pg` behind a `Queryable` interface (chosen).
+
+### Rejected
+
+- ORM — adds tooling and a codegen step for a two-statement surface; the raw SQL is reviewable and the interface keeps tests database-free.
+
+## Consequences
+
+- `InMemoryUserStore` and `PostgresUserStore` share the same async `UserStore` contract; swapping is env-driven.
+- Auth data is durable when `DATABASE_URL` is configured; the migration must be applied before first use.