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/server.go

@@ -1,5 +1,4 @@
-// Server: the HTTP listener and the background lease reaper, both shut down
-// cleanly on a signal.
+// Server: the HTTP listener, shut down cleanly on a signal.
 package infra
 
 import (
@@ -12,7 +11,7 @@ import (
 
 const shutdownGrace = 15 * time.Second
 
-// Run serves handler until ctx is cancelled, then drains in-flight requests.
+// RunServer serves handler until ctx is cancelled, then drains in-flight requests.
 func RunServer(ctx context.Context, log *slog.Logger, addr string, handler http.Handler) error {
 	srv := &http.Server{
 		Addr:              addr,
@@ -43,32 +42,3 @@ func RunServer(ctx context.Context, log *slog.Logger, addr string, handler http.
 	defer cancel()
 	return srv.Shutdown(shutdownCtx)
 }
-
-// RunReaper periodically reclaims tasks whose lease elapsed, so a worker that
-// died without a heartbeat cannot strand its task in 'leased' forever.
-// RunPeriodic invokes fn on an interval until ctx is done, logging how many rows
-// each tick affected. It backs the background reapers (expired leases, offline
-// workers) — each is a set-based UPDATE that is safe to run repeatedly and
-// concurrently across coordinators.
-func RunPeriodic(ctx context.Context, log *slog.Logger, name string, interval time.Duration,
-	fn func(context.Context) (int64, error)) {
-
-	t := time.NewTicker(interval)
-	defer t.Stop()
-
-	for {
-		select {
-		case <-ctx.Done():
-			return
-		case <-t.C:
-			n, err := fn(ctx)
-			if err != nil {
-				log.Debug(name+" skipped", "err", err)
-				continue
-			}
-			if n > 0 {
-				log.Info(name, "count", n)
-			}
-		}
-	}
-}