Add webhook logging, pagination, middleware, migrations, and prod hardening

- Webhook event logging repo and subscription webhook idempotency
- Pagination helper (echohelpers) with cursor/offset support
- Request ID and structured logging middleware
- Push client improvements (FCM HTTP v1, better error handling)
- Task model version column, business constraint migrations, targeted indexes
- Expanded categorization chain tests
- Email service and config hardening
- CI workflow updates, .gitignore additions, .env.example updates

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

internal/models/task.go

@@ -85,6 +85,9 @@ type Task struct {
 	IsCancelled   bool             `gorm:"column:is_cancelled;default:false;index" json:"is_cancelled"`
 	IsArchived    bool             `gorm:"column:is_archived;default:false;index" json:"is_archived"`
 
+	// Optimistic locking version
+	Version       int              `gorm:"column:version;not null;default:1" json:"-"`
+
 	// Parent task for recurring tasks
 	ParentTaskID  *uint            `gorm:"column:parent_task_id;index" json:"parent_task_id"`
 	ParentTask    *Task            `gorm:"foreignKey:ParentTaskID" json:"parent_task,omitempty"`