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/grpcserver/credentialservice.go

@@ -47,7 +47,11 @@ func (c *credentialServiceServer) GetPGCreds(ctx context.Context, req *mciasv1.G
 	}
 
 	// Decrypt the password for admin retrieval.
-	password, err := crypto.OpenAESGCM(c.s.masterKey, cred.PGPasswordNonce, cred.PGPasswordEnc)
+	masterKey, mkErr := c.s.vault.MasterKey()
+	if mkErr != nil {
+		return nil, status.Error(codes.Unavailable, "vault sealed")
+	}
+	password, err := crypto.OpenAESGCM(masterKey, cred.PGPasswordNonce, cred.PGPasswordEnc)
 	if err != nil {
 		return nil, status.Error(codes.Internal, "internal error")
 	}
@@ -94,7 +98,11 @@ func (c *credentialServiceServer) SetPGCreds(ctx context.Context, req *mciasv1.S
 		return nil, status.Error(codes.Internal, "internal error")
 	}
 
-	enc, nonce, err := crypto.SealAESGCM(c.s.masterKey, []byte(cr.Password))
+	masterKey, mkErr := c.s.vault.MasterKey()
+	if mkErr != nil {
+		return nil, status.Error(codes.Unavailable, "vault sealed")
+	}
+	enc, nonce, err := crypto.SealAESGCM(masterKey, []byte(cr.Password))
 	if err != nil {
 		return nil, status.Error(codes.Internal, "internal error")
 	}