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/inttest/email_test.go

@@ -62,7 +62,7 @@ func TestEmailConfirmFlow(t *testing.T) {
 }
 
 // TestEmailAlreadyTakenByAnotherAccount refuses to bind an email confirmed by a
-// different account (merge is a later stage).
+// different account (combining two accounts is the separate link/merge flow).
 func TestEmailAlreadyTakenByAnotherAccount(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)