feat: customer-level credentials shared across all nodes (item 2)

One credential set per customer (UUID + 32-byte root relay key + shortId)
lives on the customer record; node bindings no longer store credentials.
SS2022 user keys derive deterministically from the root key by method
length (16/24/32-byte prefix), so a single password serves every protocol,
matching 3x-ui's per-client credential model. Reality public keys stay a
node property and are shown per binding in the credentials pane.

- store: load-time convergence — first binding's set wins; a converged
  password that is not a 32-byte root key is regenerated (uuid/shortId
  preserved); bindings are stripped; idempotent, covered by tests
- deploy: the orchestrator stamps the customer's set onto a build copy;
  subscriptions resolve the same set; the derivation applies in both the
  inbound builder and the URI builder (verified live: Alice's N1 link is
  byte-identical to the pre-change seed expectation, her N2 user key is
  exactly the root key's 16-byte prefix)
- API: GET /api/customers/:id/credentials returns {credentials, bindings};
  regenerate is now POST /api/customers/:id/credentials/regenerate and
  rotates the whole set, rebuilding every deployed node of the customer
- UI: credentials pane shows the single shared set plus per-binding
  node/protocol/Reality-public-key rows
- golden subscriptions fixture regenerated under one shared set (RF and
  SS entries byte-identical to before; five entries audited by hand)
- golden baseline re-captured for the new shapes; difftest normalizer now
  treats a base64 root key's prefix-truncation encodings and the Surge
  '='-mangled variants as the same secret

panel-go/types.go

@@ -560,9 +560,13 @@ type Customer struct {
 	ServerUsage          map[string]*UsageSlot `json:"serverUsage,omitempty"`
 	LastUsageAt          *string               `json:"lastUsageAt,omitempty"`
 	LastOnlineAt         *string               `json:"lastOnlineAt,omitempty"`
-	ExternalSources      []ExtSource           `json:"externalSources,omitempty"`
-	ManualLinks          []ManualLink          `json:"manualLinks,omitempty"`
-	Extra                extraFields           `json:"-"`
+	// The customer's single credential set, shared by all of their node
+	// bindings (round-2 item 2). Populated by the store-load migration for
+	// legacy records and at creation for new ones.
+	Credentials     *Credentials `json:"credentials,omitempty"`
+	ExternalSources []ExtSource  `json:"externalSources,omitempty"`
+	ManualLinks     []ManualLink `json:"manualLinks,omitempty"`
+	Extra           extraFields  `json:"-"`
 }
 
 // ExtSource is one external subscription source on a customer
@@ -781,14 +785,18 @@ func (c Credentials) MarshalJSON() ([]byte, error) {
 }
 
 type NodeClient struct {
-	ID          string      `json:"id"`
-	NodeID      string      `json:"nodeId"`
-	CustomerID  string      `json:"customerId"`
-	Credentials Credentials `json:"credentials"`
-	Status      string      `json:"status"`
-	CreatedAt   string      `json:"createdAt"`
-	UpdatedAt   string      `json:"updatedAt"`
-	Extra       extraFields `json:"-"`
+	ID         string `json:"id"`
+	NodeID     string `json:"nodeId"`
+	CustomerID string `json:"customerId"`
+	// Deprecated: credentials live on the Customer (round-2 item 2). This
+	// field is only a migration input — the store-load migration converges
+	// it into Customer.Credentials and clears it, so it disappears from
+	// disk on the next save. Builders receive resolved copies instead.
+	Credentials *Credentials `json:"credentials,omitempty"`
+	Status      string       `json:"status"`
+	CreatedAt   string       `json:"createdAt"`
+	UpdatedAt   string       `json:"updatedAt"`
+	Extra       extraFields  `json:"-"`
 }
 
 func (n *NodeClient) UnmarshalJSON(raw []byte) error {