Add vault seal/unseal lifecycle

- New internal/vault package: thread-safe Vault struct with
  seal/unseal state, key material zeroing, and key derivation
- REST: POST /v1/vault/unseal, POST /v1/vault/seal,
  GET /v1/vault/status; health returns sealed status
- UI: /unseal page with passphrase form, redirect when sealed
- gRPC: sealedInterceptor rejects RPCs when sealed
- Middleware: RequireUnsealed blocks all routes except exempt
  paths; RequireAuth reads pubkey from vault at request time
- Startup: server starts sealed when passphrase unavailable
- All servers share single *vault.Vault by pointer
- CSRF manager derives key lazily from vault

Security: Key material is zeroed on seal. Sealed middleware
runs before auth. Handlers fail closed if vault becomes sealed
mid-request. Unseal endpoint is rate-limited (3/s burst 5).
No CSRF on unseal page (no session to protect; chicken-and-egg
with master key). Passphrase never logged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/vault/vault.go

@@ -0,0 +1,127 @@
+// Package vault provides a thread-safe container for the server's
+// cryptographic key material with seal/unseal lifecycle management.
+//
+// Security design:
+//   - The Vault holds the master encryption key and Ed25519 signing key pair.
+//   - All accessors return ErrSealed when the vault is sealed, ensuring that
+//     callers cannot use key material that has been zeroed.
+//   - Seal() explicitly zeroes all key material before nilling the slices,
+//     reducing the window in which secrets remain in memory after seal.
+//   - All state transitions are protected by sync.RWMutex. Readers (IsSealed,
+//     MasterKey, PrivKey, PubKey) take a read lock; writers (Seal, Unseal)
+//     take a write lock.
+package vault
+
+import (
+	"crypto/ed25519"
+	"errors"
+	"sync"
+)
+
+// ErrSealed is returned by accessor methods when the vault is sealed.
+var ErrSealed = errors.New("vault is sealed")
+
+// Vault holds the server's cryptographic key material behind a mutex.
+// All three servers (REST, UI, gRPC) share a single Vault by pointer.
+type Vault struct {
+	mu        sync.RWMutex
+	masterKey []byte
+	privKey   ed25519.PrivateKey
+	pubKey    ed25519.PublicKey
+	sealed    bool
+}
+
+// NewSealed creates a Vault in the sealed state. No key material is held.
+func NewSealed() *Vault {
+	return &Vault{sealed: true}
+}
+
+// NewUnsealed creates a Vault in the unsealed state with the given key material.
+// This is the backward-compatible path used when the passphrase is available at
+// startup.
+func NewUnsealed(masterKey []byte, privKey ed25519.PrivateKey, pubKey ed25519.PublicKey) *Vault {
+	return &Vault{
+		masterKey: masterKey,
+		privKey:   privKey,
+		pubKey:    pubKey,
+		sealed:    false,
+	}
+}
+
+// IsSealed reports whether the vault is currently sealed.
+func (v *Vault) IsSealed() bool {
+	v.mu.RLock()
+	defer v.mu.RUnlock()
+	return v.sealed
+}
+
+// MasterKey returns the master encryption key, or ErrSealed if sealed.
+func (v *Vault) MasterKey() ([]byte, error) {
+	v.mu.RLock()
+	defer v.mu.RUnlock()
+	if v.sealed {
+		return nil, ErrSealed
+	}
+	return v.masterKey, nil
+}
+
+// PrivKey returns the Ed25519 private signing key, or ErrSealed if sealed.
+func (v *Vault) PrivKey() (ed25519.PrivateKey, error) {
+	v.mu.RLock()
+	defer v.mu.RUnlock()
+	if v.sealed {
+		return nil, ErrSealed
+	}
+	return v.privKey, nil
+}
+
+// PubKey returns the Ed25519 public key, or ErrSealed if sealed.
+func (v *Vault) PubKey() (ed25519.PublicKey, error) {
+	v.mu.RLock()
+	defer v.mu.RUnlock()
+	if v.sealed {
+		return nil, ErrSealed
+	}
+	return v.pubKey, nil
+}
+
+// Unseal transitions the vault from sealed to unsealed, storing the provided
+// key material. Returns an error if the vault is already unsealed.
+func (v *Vault) Unseal(masterKey []byte, privKey ed25519.PrivateKey, pubKey ed25519.PublicKey) error {
+	v.mu.Lock()
+	defer v.mu.Unlock()
+	if !v.sealed {
+		return errors.New("vault is already unsealed")
+	}
+	v.masterKey = masterKey
+	v.privKey = privKey
+	v.pubKey = pubKey
+	v.sealed = false
+	return nil
+}
+
+// Seal transitions the vault from unsealed to sealed. All key material is
+// zeroed before being released to minimize the window of memory exposure.
+//
+// Security: explicit zeroing loops ensure the key bytes are overwritten even
+// if the garbage collector has not yet reclaimed the backing arrays.
+func (v *Vault) Seal() {
+	v.mu.Lock()
+	defer v.mu.Unlock()
+	// Zero master key.
+	for i := range v.masterKey {
+		v.masterKey[i] = 0
+	}
+	v.masterKey = nil
+	// Zero private key.
+	for i := range v.privKey {
+		v.privKey[i] = 0
+	}
+	v.privKey = nil
+	// Zero public key (not secret, but consistent cleanup).
+	for i := range v.pubKey {
+		v.pubKey[i] = 0
+	}
+	v.pubKey = nil
+	v.sealed = true
+}