chore(users): remove coordinator leftovers, tidy for userservice

- delete api/requests.http and ARCHITECTURE.md (coordinator content)
- rewrite README.md and .env.example for the userservice
- drop dead RunPeriodic (reaper machinery userservice has no use for)
- fix .gitignore/.dockerignore/.golangci.yml module + artifact names
- degeneralize stale copied comments that said "coordinator"

users/internal/infra/clock.go

@@ -8,6 +8,6 @@ type System struct{}
 
 func NewClock() System { return System{} }
 
-// Now returns UTC so every timestamp the coordinator writes is comparable
+// Now returns UTC so every timestamp this service writes is comparable
 // regardless of the host's timezone.
 func (System) Now() time.Time { return time.Now().UTC() }