feat: harden support, localize knowledge and add operator availability [skip ci]

Source-only publication of reviewed commit 64e96a7f423673dc29ad56485be9fc79bce8af1c.

Passed in isolation: focused frontend/backend tests, race checks, TypeScript typecheck, lint, Go compilation, Flowgram build, and deterministic SDK build/tests.

Not accepted as a production release: Next.js export/native assembly remains blocked by environment memory limits; browser interaction and MySQL staging acceptance remain unverified. No production server, database/schema, runtime configuration, deployment, release tag, or workflow execution was performed.

Expected source tree: 0a4a7528e9.

internal/pkg/dto/dto.go

@@ -1,6 +1,11 @@
+// Modified in the independent hardening fork; see docs/hardening.
+
 package dto
 
-import "agent-desk/internal/pkg/enums"
+import (
+	"agent-desk/internal/pkg/enums"
+	"time"
+)
 
 type AuthPrincipal struct {
 	UserID      int64
@@ -11,16 +16,22 @@ type AuthPrincipal struct {
 	Status      enums.Status
 	Roles       []string
 	Permissions []string
+
+	// Session metadata stays internal and binds long-lived connections to the
+	// original login session and its absolute deadline.
+	LoginSessionID int64     `json:"-"`
+	ExpiresAt      time.Time `json:"-"`
 }
 
 type WebChannelConfig struct {
-	Title           string   `json:"title"`
-	Subtitle        string   `json:"subtitle"`
-	ThemeColor      string   `json:"themeColor"`
-	Position        string   `json:"position"`
-	Width           string   `json:"width"`
-	UserTokenSecret string   `json:"userTokenSecret,omitempty"`
-	PromptStarters  []string `json:"promptStarters,omitempty"`
+	Title                 string   `json:"title"`
+	Subtitle              string   `json:"subtitle"`
+	ThemeColor            string   `json:"themeColor"`
+	Position              string   `json:"position"`
+	Width                 string   `json:"width"`
+	UserTokenSecret       string   `json:"userTokenSecret,omitempty"`
+	RequireSignedIdentity bool     `json:"requireSignedIdentity,omitempty"`
+	PromptStarters        []string `json:"promptStarters,omitempty"`
 }
 
 type TelegramChannelConfig struct {