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.

backend/internal/game/doc.go

@@ -16,5 +16,5 @@
 // word-check tool with complaint capture, per-player game state, history and GCG
 // export, and the per-game turn-timeout sweeper that auto-resigns an overdue
 // player (honouring their daily away window). The HTTP surface that fronts these
-// operations is added with the gateway in a later stage.
+// operations is exposed to the gateway.
 package game

backend/internal/game/types.go

@@ -105,7 +105,7 @@ const MaxActiveQuickGames = 10
 const aiPlayerName = "AI"
 
 // CreateParams describes a new game. Seats lists the seated accounts in turn
-// order (seat 0 moves first); lobby/matchmaking assembles it in a later stage.
+// order (seat 0 moves first); lobby/matchmaking assembles it.
 type CreateParams struct {
 	Variant        engine.Variant
 	Seats          []uuid.UUID