Stage 0: scaffold monorepo, backend skeleton, docs, CI

- go.work (Go 1.26.3) with backend module; deps added incrementally (gin+zap only)

- backend: /healthz + /readyz, env config, graceful shutdown

- docs: ARCHITECTURE, FUNCTIONAL (+ru mirror), TESTING

- PLAN.md (stage tracker + per-stage open details) and CLAUDE.md (per-stage workflow)

- .gitea go-unit CI (gofmt/vet/build/test)

docs/TESTING.md

@@ -0,0 +1,37 @@
+# Scrabble Game — Testing
+
+How the project is tested and the gate every stage must pass. Read before adding
+tests or touching CI.
+
+## Layers
+
+- **Go unit tests** — table-driven where it helps; `testing` + standard library.
+  Every functional change ships with regression coverage. Run:
+  `go test -count=1 ./backend/...` (the module list grows with the workspace).
+- **Integration** *(introduced with Postgres in Stage 1)* — `testcontainers-go`
+  spins real dependencies (Postgres). Slow; a separate CI workflow.
+- **UI** *(introduced with the UI in Stage 7)* — Vitest (unit) + Playwright
+  (e2e), mirroring the chosen plain-Svelte + Vite toolchain.
+- **Engine** — correctness of scoring and move generation is owned by
+  `scrabble-solver`'s own GCG-backed tests. The backend adds regression tests
+  for end-conditions, the 24-hour timeout / auto-resign, robot balance and
+  margin targeting, and **dictionary-independent history replay**.
+
+## Principles
+
+- A green run must not depend on cached state: use `-count=1` in CI.
+- Tests that need infrastructure fail loudly (`t.Fatal`) when it is unavailable
+  rather than silently skipping coverage.
+- No network or real platform calls in unit tests; validate platform
+  credentials behind an interface seam and test with fixtures.
+
+## Per-stage CI gate
+
+Every completed stage is exercised on `gitea.iliadenisov.ru` before it is marked
+done in [`../PLAN.md`](../PLAN.md):
+
+1. Commit the stage on its `feature/*` branch.
+2. Push to `origin`.
+3. Watch the run to completion — never hand-roll a poll loop:
+   `python3 ~/.claude/bin/gitea-ci-watch.py` (launch in the background).
+4. Only after every workflow that fired is green may the stage be marked done.