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/server/server.go

@@ -1,10 +1,10 @@
 // Package server wires the backend's HTTP listener: the gin engine, its route
 // groups, the per-request telemetry middleware and the start/stop lifecycle.
 //
-// The /api/v1 route groups (public, user, internal, admin) are created here so
-// later stages attach their endpoints to a stable structure; the /user group
-// requires the X-User-ID identity header. The probes /healthz (liveness) and
-// /readyz (database + session-cache readiness) are unauthenticated.
+// The /api/v1 route groups (public, user, internal, admin) attach their endpoints
+// to a stable structure; the /user group requires the X-User-ID identity header.
+// The probes /healthz (liveness) and /readyz (database + session-cache readiness)
+// are unauthenticated.
 package server
 
 import (
@@ -245,7 +245,7 @@ func (s *Server) Invitations() *lobby.InvitationService { return s.invitations }
 func (s *Server) Emails() *account.EmailService { return s.emails }
 
 // Handler returns the underlying HTTP handler. It lets tests drive the server
-// without binding a socket and lets later stages compose the backend behind
+// without binding a socket and lets callers compose the backend behind
 // another listener.
 func (s *Server) Handler() http.Handler { return s.http.Handler }