docs: finalize documentation to the production state

The project is live in production, so the staged-development scaffolding is removed.

- Delete the staged trackers PLAN.md and PRERELEASE.md.
- Rewrite CLAUDE.md: drop the per-stage workflow; codify the ongoing development
  principles (How we work) and the production model (Branching, CI & production):
  manual prod-deploy / prod-rollback, semver release tags, Ansible provisioning,
  expand-contract migrations.
- De-stage the living docs (README, ARCHITECTURE, TESTING, deploy/ansible, loadtest,
  platform/telegram READMEs) and the docker-compose tuning comments: drop the
  Stage N / R1-R7 / pre-release labels, keep every number and rationale, and fix the
  now-dangling PLAN.md / PRERELEASE.md references to describe the current state.
- Reword stale 'later stage' Go doc comments for subsystems that have shipped.

README.md

@@ -22,9 +22,8 @@ supports English Scrabble, Russian Scrabble and Эрудит.
   security, cross-service contracts.
 - [`docs/FUNCTIONAL.md`](docs/FUNCTIONAL.md) (+ [`_ru`](docs/FUNCTIONAL_ru.md)) —
   per-domain user stories.
-- [`docs/TESTING.md`](docs/TESTING.md) — test layers and the per-stage CI gate.
-- [`PLAN.md`](PLAN.md) — the staged implementation plan and stage tracker.
-- [`CLAUDE.md`](CLAUDE.md) — project guide and the mandatory per-stage workflow.
+- [`docs/TESTING.md`](docs/TESTING.md) — test layers and the CI gate.
+- [`CLAUDE.md`](CLAUDE.md) — project guide and development workflow.
 
 ## Build & test