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/models/support_availability.go

@@ -0,0 +1,24 @@
+package models
+
+import "time"
+
+// SupportCalendar is opt-in. Absence of a team row preserves dated schedules.
+// TeamID zero stores the deployment-wide timezone, not another organization.
+type SupportCalendar struct {
+	ID             int64  `gorm:"primaryKey;autoIncrement"`
+	TeamID         int64  `gorm:"not null;uniqueIndex"`
+	Mode           string `gorm:"type:varchar(20);not null;default:'legacy'"`
+	Timezone       string `gorm:"type:varchar(100);not null;default:''"`
+	WeeklyJSON     string `gorm:"type:text;not null"`
+	ExceptionsJSON string `gorm:"type:text;not null"`
+	UpdatedAt      time.Time
+}
+
+// AgentPresenceLease contains no bearer credential or client address. Multiple
+// independent connections may lease presence for the same authenticated user.
+type AgentPresenceLease struct {
+	ID             string    `gorm:"type:varchar(64);primaryKey"`
+	UserID         int64     `gorm:"not null;index"`
+	LoginSessionID int64     `gorm:"not null;index"`
+	ExpiresAt      time.Time `gorm:"not null;index"`
+}