feat(core): bind request services and implement registered tasks

Wire plugin services through Bind/InjectFrom and AppContext so HTTP and
workers resolve dependencies after Apply. Register TaskHandler objects
with persisted results, and implement send_email_code, mail:send,
cleanup_inactive_users, and dispatch_bot_msg.

backend/core/contracts/task.go

@@ -43,6 +43,12 @@ type TaskResultDTO struct {
 	Detail  any    `json:"detail,omitempty"`
 }
 
+// TaskHandler is the preferred background task handler. Drivers invoke Execute
+// and persist Message/Detail onto the execution record.
+type TaskHandler interface {
+	Execute(ctx context.Context, payload []byte) (*TaskResultDTO, error)
+}
+
 // TaskExecutionDTO represents a single task execution record.
 type TaskExecutionDTO struct {
 	ID           uint64     `json:"id,string"`