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/ui/handlers_accounts.go

@@ -460,7 +460,12 @@ func (u *UIServer) handleSetPGCreds(w http.ResponseWriter, r *http.Request) {
 	// Security: encrypt the password with AES-256-GCM before storage.
 	// A fresh random nonce is generated per call by SealAESGCM; nonce reuse
 	// is not possible. The plaintext password is not retained after this call.
-	enc, nonce, err := crypto.SealAESGCM(u.masterKey, []byte(password))
+	masterKey, err := u.vault.MasterKey()
+	if err != nil {
+		u.renderError(w, r, http.StatusInternalServerError, "internal error")
+		return
+	}
+	enc, nonce, err := crypto.SealAESGCM(masterKey, []byte(password))
 	if err != nil {
 		u.logger.Error("encrypt pg password", "error", err)
 		u.renderError(w, r, http.StatusInternalServerError, "internal error")
@@ -864,7 +869,12 @@ func (u *UIServer) handleCreatePGCreds(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Security: encrypt with AES-256-GCM; fresh nonce per call.
-	enc, nonce, err := crypto.SealAESGCM(u.masterKey, []byte(password))
+	masterKey, err := u.vault.MasterKey()
+	if err != nil {
+		u.renderError(w, r, http.StatusInternalServerError, "internal error")
+		return
+	}
+	enc, nonce, err := crypto.SealAESGCM(masterKey, []byte(password))
 	if err != nil {
 		u.logger.Error("encrypt pg password", "error", err)
 		u.renderError(w, r, http.StatusInternalServerError, "internal error")