adminauth: a break-glass local admin alongside OIDC

If the IdP is misconfigured, unreachable, or the admin group is a typo, the
operator is locked out of their own server with no way back short of editing
JSON on disk. A fallback that only matters when everything else is broken is
exactly the thing you cannot add later - by then you cannot get in to add it.

Stored as PBKDF2-HMAC-SHA256 from the standard library (Go 1.24+ has it, so no
dependency), 600k iterations, per-credential salt. A password rather than a
bearer token on purpose: a break-glass credential is the one most likely to end
up in a backup or a config-management repo, and a hash survives that where a
token does not. There is no email reset flow and should not be -
--set-admin-password on the host is the reset, and whoever can run it already
has the machine.

The password is read from stdin, never a flag, so it stays out of shell history
and the process list; piping still works for automation.

Details the tests pin, each for a reason:
  - the username is compared in constant time too, or a fast rejection is a
    timing oracle for which usernames exist;
  - the *stored* iteration count is used, so raising the constant later does not
    lock out existing passwords;
  - the throttle grows with consecutive failures but stays bounded and forgives
    after a quiet minute - a break-glass credential an attacker can lock out is
    a denial of service against the one person who needs it;
  - sessions are MAC-checked before anything in them is read, and rotating the
    secret invalidates every one at once, which is how they are revoked.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/internal/store/store.go

@@ -16,6 +16,8 @@ import (
 	"os"
 	"path/filepath"
 	"sync"
+
+	"echo-lot.app/server/internal/adminauth"
 	"time"
 )
 
@@ -59,6 +61,12 @@ type Store struct {
 type fileData struct {
 	Tokens  []EnrollToken `json:"tokens"`
 	Devices []Device      `json:"devices"`
+	// The break-glass admin. Absent until an operator sets one.
+	LocalAdmin *adminauth.Credential `json:"local_admin,omitempty"`
+	// Signing secret for admin session cookies. Persisted so sessions survive a restart;
+	// deleting it from the state file invalidates every session at once, which is how an
+	// operator revokes them.
+	SessionSecret string `json:"session_secret,omitempty"`
 }
 
 func Open(stateDir string) (*Store, error) {
@@ -177,6 +185,50 @@ func (s *Store) DeleteDevice(id string) error {
 	return errors.New("no such device")
 }
 
+// SetLocalAdmin stores (or replaces) the break-glass admin password.
+func (s *Store) SetLocalAdmin(c adminauth.Credential) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.data.LocalAdmin = &c
+	return s.save()
+}
+
+// LocalAdmin returns the configured break-glass admin, or nil.
+func (s *Store) LocalAdmin() *adminauth.Credential {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.data.LocalAdmin == nil {
+		return nil
+	}
+	c := *s.data.LocalAdmin
+	return &c
+}
+
+// ClearLocalAdmin removes the break-glass admin.
+func (s *Store) ClearLocalAdmin() error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.data.LocalAdmin = nil
+	return s.save()
+}
+
+// SessionSecret returns the admin session signing secret, creating one on first use.
+func (s *Store) SessionSecret() ([]byte, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.data.SessionSecret != "" {
+		if b, err := hex.DecodeString(s.data.SessionSecret); err == nil && len(b) >= 32 {
+			return b, nil
+		}
+	}
+	b, err := adminauth.NewSecret()
+	if err != nil {
+		return nil, err
+	}
+	s.data.SessionSecret = hex.EncodeToString(b)
+	return b, s.save()
+}
+
 func (s *Store) DeviceByCredential(cred string) *Device {
 	s.mu.Lock()
 	defer s.mu.Unlock()