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/social/social.go

@@ -3,8 +3,8 @@
 // in as a message kind. It owns the friendships, blocks and chat_messages tables,
 // reads the account-level block toggles through account.Store, and gates chat and
 // nudge on game state through a GameReader so it never imports the engine. The
-// live delivery of chat and nudges (push / in-app stream) belongs to the gateway
-// in a later stage; this package only persists and reads them.
+// live delivery of chat and nudges (push / in-app stream) belongs to the gateway;
+// this package only persists and reads them.
 package social
 
 import (