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/upgrade.go

@@ -242,7 +242,7 @@ func upgradeCopy(ctx context.Context, r *os.Root, source, target string, limit i
 	return upgradeSync(r)
 }
 
-// Call while holding a.mu before any operation that can start or mutate a runtime.
+// Call while holding the instance lock before any operation that can start or mutate a runtime.
 func (a *API) upgradeRecoveryRequired(i store.Instance) bool {
 	var exists int
 	if err := a.s.DB.QueryRow("SELECT count(*) FROM sqlite_master WHERE type='table' AND name='upgrade_journal'").Scan(&exists); err != nil {
@@ -275,14 +275,17 @@ func (a *API) upgrade(w http.ResponseWriter, r *http.Request, i store.Instance)
 		fail(w, 400, "invalid image_id")
 		return
 	}
-	a.mu.Lock()
-	defer a.mu.Unlock()
-	if a.workspaceReaders[i.ID] > 0 {
+	inst := a.instanceLocks.Acquire(i.ID)
+	defer inst.Release()
+	a.readersMu.Lock()
+	readers := a.workspaceReaders[i.ID]
+	a.readersMu.Unlock()
+	if readers > 0 {
 		fail(w, 409, "instance files busy; retry later")
 		return
 	}
-	workspaceMu.Lock()
-	defer workspaceMu.Unlock()
+	user := a.userLocks.Acquire(i.UID)
+	defer user.Release()
 	current, err := a.s.GetInstance(i.ID)
 	if err != nil || current.UID != i.UID {
 		fail(w, 404, "not found")