fix(api-server): mark session_messages seq as read-only in Gorm model

The refactor from raw SQL to g2.Create(msg) in PR #1660 caused Gorm
to include seq=0 in INSERT statements, colliding with the unique
constraint when a seq=0 row already existed. The -> tag tells Gorm
to omit seq from writes and let the Postgres DEFAULT nextval() fire,
while still reading the value back via the RETURNING clause.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jsell-rh

components/ambient-api-server/plugins/sessions/message_model.go

@@ -5,7 +5,7 @@ import "time"
 type SessionMessage struct {
 	ID        string    `gorm:"column:id;primaryKey;type:varchar(36)" json:"id"`
 	SessionID string    `gorm:"column:session_id;type:varchar(36)" json:"session_id"`
-	Seq       int64     `gorm:"column:seq" json:"seq"`
+	Seq       int64     `gorm:"column:seq;->" json:"seq"`
 	EventType string    `gorm:"column:event_type;type:varchar(255)" json:"event_type"`
 	Payload   string    `gorm:"column:payload;type:text" json:"payload"`
 	CreatedAt time.Time `gorm:"column:created_at;type:timestamptz" json:"created_at"`