Fix audit findings: lock architecture, security, deployment hardening

Close all 14 findings from the codebase audit (1 high, 5 medium, 8 low):

- Locking: replace the global API and workspace mutexes with per-instance
  and per-user keyed locks (new internal/api/locks.go) so a slow restore,
  upgrade or transfer no longer blocks unrelated users or instances; move
  Docker I/O out of the build queue and invalidate concurrent image-index
  observations with a generation counter; serialize instance deletion
  behind in-flight creation.
- Security: equalize login timing for unknown usernames, periodically
  purge expired sessions/refresh tokens/lock rows, URL-encode orphan IDs
  in the frontend, upgrade vitest 3.x->5.x to clear 3 npm audit
  vulnerabilities (2 critical RCE).
- Deployment: add a -healthcheck mode probing /healthz (used by Dockerfile
  HEALTHCHECK and compose), make ADMIN_PASSWORD optional in compose with
  removal guidance after first start, exclude test scripts from the image,
  and cap panel resources (cpus: 2, pids_limit: 512).
- Reliability: check rows.Err() on admin listing loops, surface intent
  polling errors without sticking, disable close during in-flight create.

Verified: go build/vet, full go test -race -count=1 ./..., frontend
tsc build + 32 tests, npm audit 0, compose config, real-container
healthcheck.

internal/api/orphans.go

@@ -101,8 +101,9 @@ func (a *API) orphans(w http.ResponseWriter, r *http.Request) {
 	}
 	ctx, cancel := context.WithTimeout(r.Context(), 20*time.Second)
 	defer cancel()
-	a.mu.Lock()
-	defer a.mu.Unlock()
+	// checkOrphan re-verifies instance rows, upgrade journal entries and mount
+	// identities before removal, so no lifecycle lock is needed here: a
+	// container is only reclaimed when its instance row is already gone.
 	switch r.Method {
 	case "GET":
 		filters, _ := json.Marshal(map[string][]string{"label": {"dsh-mgr.role=dsh"}})